Why Did House Tully Join Rebellion, Really ?

[Melpomene]

It's probably a geographical reason too...the Riverlands are in the center of the continent, leading the area to be a common location for battles (Battle of the Trident itself, was in the Riverlands) And additionally, because of their location, they are weaker defensively.

Perhaps it is just me, but with such a central location, and a very vulnerable one regardless of great economy or stability in peacetime, I would opt for siding with the most logical folks in terms of locale too, especially if there was already an alliance via marriage ahead of time with those said locales. Edmure was one to think about his commonfolk, perhaps that was influenced by Hoster himself? Overall, all the houses of the North, most houses of the Vale not far off in the East, Lannisters and Baratheons down south, House Frey in the Riverlands also but completely inconclusive/unpredictable in sides - It might be tricky to be the rebels enemy, would definitely get a good ol' beating from all ends, directions and angles, and most of the brunt of it all on behalf of a royal cause.

But that is just a guess. Perhaps Hoster wasn't thinking that, and was mainly thinking on the current deal, and the future success if the Rebellion panned out well. Aerys was a nutcase, worthy of worry and anger - imagine if he had continued living and ruling. Just to try and pick at the guys brain... if you were Hoster Tully, would you be concerned about your kids marrrying into or close to that family in some way, if opting against the others? And if there was a marriage alliance that was connected with the Targs or their houses, would it feel unstable or worrisome, knowing that the Targ king killed a previous betrothed and risk slicing away that important alliance without permission or thought? It was lucky that Brandon did have other brothers to renew the alliance with. Also, if Aerys was willing to just torture and kill two in-laws from the same family, who knows what he could decide on Hoster's own son and daughters in the long run. Aerys was known to be harsh with his own wife, be aroused by fire, and try to take liberties with other women even if they were sworn to another, conducted cruel punishments on anyone for very minor offences, and so forth. Aerys was also known to be jealous, and would scoff off any other marriages purely based on that jealousy. Aerys also destroyed two houses prior, brutally.

He likely knew, of course, that the likes of Arryn and Stark were not chopped liver, made to face ruin and loss. To lose a bunch of these big houses could have dire consequences. The lord of the now destroyed house could have had a reasonable cause for rebelling then, and now that it is technically happening again with other houses showing their displeasure...might be a reason to go forth and try and stop the king.

Siding with the Targ cause would be its own high risks. There was no real definite security with them, even if they won. No security with a stable marital agreement, and his lands would likely be muuuuch worse off in comparison to being with the rebel league. Economy and connections could dwindle significantly. If Aerys lived, the problems would be there, if Rhaegar succeeded in whatever plans he had, there was no garauntee with him either. Rhaegar helped create the war in many people's eyes, people witnessed him fight against rebels, and even if he was dandy, smart and sane, he could go nuts too. Aerys was dandy, smart and sane in his younger years too.

And even if Hoster Tully and the rebels failed against the loyalists, it might be still better to fight on behalf of your choice/cause/household rights and die for those, instead of dying under the fearfully tightened leash and rule of a despicable, unstable and harsh person that you felt uneasy about for years now. Perhaps there was more honour in the long run for the former instead of the latter.

Hoster Tully decisions must have meant something, definitely, with serious consideration and quick thinking, to choose the rebels. He even rid of his bannermen who were pro Targ.

[Small Bucket]

The Tullys had a greater responsibility to the people of the Riverlands and their sworn Lords than they did to their liege Lords.

The Targs were kidnapping ladies and murdering Lords, in violation of both their oaths, the law and chivalry. There was no reason to believe they wouldn't be next.

"Thanks for helping put down that rebellion, Hoster. Now come here so I can reward you by setting your children on fire!"

To protect his family, for the duty owed his people and for the honor he swore before the Seven, Hoster Tully had no choice but to take up arms for justice against his King.

[Victor227]

Soft-hearted concern for the smallfolk was definitely not it. Hoster was every bit a butcher, rolling through with fire and sword on villages in the Riverlands when their lords failed to support the rebellion.

[mindchap]

Lysa, pure and simple. He gives his spears and gains not only a husband for a soiled daughter but a husband that is one of the most powerful lords in Westeros. One of the four wardens makes a powerful match.

[Mountain Tribesman]

Several reasons motivated Tully, I believe.

-The strategic position of the Riverlands was simply better if they sided with the Rebels. Tywin was apparently neutral, so the only threat from the Royalists came from the South, whereas if they joined the Royalists, they'd quickly be assaulted from the north and the east, possibily the west as well if Tywin decided to play against Aerys (and his neutrality was a strong indicator that he'd turn against the king when he was sure of the outcome).

-Brandon, Rickard and Jeffory Mallister's deaths showed everyone that Aerys would, on a whim, murder high lords of Westerosi houses. Knowing his madness, any lord with a bit of sense knew that they could be next, including Hoster or even Tywin.

-The opportunity for political advancement, of course.

I wouldn't criticize Hoster for turning against the Targaryens, despite what Aegon had given them 300 years earlier. 300 years is a lot, and once a kingly line has gone bad, it's perfectly reasonable to want it replaced with a better one.