Theory: Littlefinger was poisoning Hoster Tully.

[Angel Eyes]

So it's known that Littlefinger engineered a war to plunge the Seven Kingdoms into chaos so that he could profit financially and power-wise. And everyone knows that the Riverlands becomes a battleground no matter what. I think that Littlefinger bribed Maester Vyman into poisoning Hoster Tully with either Widow's Blood, a poison that causes the bladder and bowels to fail, or the Tears of Lys, which eats at the bowels and belly of the victim; he dies in agony, which will not appear unusual if the victim is old or sickly. Hoster is in his elder years at this point, it's implied that the sickness he has affects his bowels. So in this move, Littlefinger puts the Riverlands into the hands of a boneheaded, impulsive heir, forces House Stark into a conflict of Littlefinger's choosing due to their marriage ties to House Tully, and Littlefinger gets revenge on Hoster Tully for betrothing Catelyn to Brandon Stark, the (admittedly stupid) move that changed Littlefinger's life.

[Lord Varys]

Not very likely. Hoster was sick and ailing and dying for months and years, and those poisons should both work much faster (and the Tears of Lys definitely do).

Instead, it really seems Hoster was dying slowly of intestinal cancer or a similar painful disease.

Natural diseases are a thing in Westeros - Visenya Targaryen also seems to have died of some form of cancer, at least that's what he growing thin and haggard in old age with the flesh melting of her bones could imply.

King Aenys may also have died naturally of some stomach disease or infection. If he was poisoned by means of the Tears of Lys the Grand Maester and Maester Culiper of Dragonstone would have seen through that immediately - not to mention that he would have been died much sooner. We know from Jon Arryn that the Tears kill very quickly and Androw Farman's poisoning spree established you die in 1-2 days after you first symptoms manifest.

[Targaryen Restoration]

That would involve planning the war even before Cat took Tyrion from the inn.  No way PB would know all those would come to pass.  He improvised with the knife to frame Tyrion.  He assumed war would happen as the result of the arrogance of the families involved.  

[Angel Eyes]

9 hours ago, Targaryen Restoration said:

That would involve planning the war even before Cat took Tyrion from the inn.  No way PB would know all those would come to pass.  He improvised with the knife to frame Tyrion.  He assumed war would happen as the result of the arrogance of the families involved.  

Well, why else did he have Lysa poison Jon Arryn? He would know that Robert would come north to name Ned Hand of the King, have Lysa send another letter implicating the Lannisters, Littlefinger would "assist" Ned in figuring out the parentage of Robert's "children" and if that news was known in any form, the Lannisters and Starks would be fighting because Tywin would not let that stand, with the Tullys caught in the crossfire.

I guess that it's hard to imagine Littlefinger not wanting revenge on Hoster for betrothing Catelyn to Brandon.

[Therae]

On 2/29/2020 at 9:50 AM, Angel Eyes said:

Well, why else did he have Lysa poison Jon Arryn? He would know that Robert would come north to name Ned Hand of the King, have Lysa send another letter implicating the Lannisters, Littlefinger would "assist" Ned in figuring out the parentage of Robert's "children" and if that news was known in any form, the Lannisters and Starks would be fighting because Tywin would not let that stand, with the Tullys caught in the crossfire.

I guess that it's hard to imagine Littlefinger not wanting revenge on Hoster for betrothing Catelyn to Brandon.

We have it on pretty good authority that Lysa had her own reasons for being happy to agree to poison Jon Arryn. We have no reason to believe that Maester Vyman would have wanted to poison Hoster. LF may well have wanted him dead, but it seems like he's pretty happy to take his revenge by way of long con, and currently, as far as he knows, nearly all the Tullys, Starks, and Arryns are either dead, disinherited, or in his personal guardianship. If revenge was his goal (I don't disagree that he wanted some), I'd say he probably believes he's sitting pretty without having contributed to Hoster's demise.

[Therae]

Adding to the above, it's actually LF's trademark to have accomplices with their own motivations, who actually do most of the dirty work. We see that over and over with Littlefinger.

[John Suburbs]

On 2/28/2020 at 9:08 PM, Angel Eyes said:

So it's known that Littlefinger engineered a war to plunge the Seven Kingdoms into chaos so that he could profit financially and power-wise. And everyone knows that the Riverlands becomes a battleground no matter what. I think that Littlefinger bribed Maester Vyman into poisoning Hoster Tully with either Widow's Blood, a poison that causes the bladder and bowels to fail, or the Tears of Lys, which eats at the bowels and belly of the victim; he dies in agony, which will not appear unusual if the victim is old or sickly. Hoster is in his elder years at this point, it's implied that the sickness he has affects his bowels. So in this move, Littlefinger puts the Riverlands into the hands of a boneheaded, impulsive heir, forces House Stark into a conflict of Littlefinger's choosing due to their marriage ties to House Tully, and Littlefinger gets revenge on Hoster Tully for betrothing Catelyn to Brandon Stark, the (admittedly stupid) move that changed Littlefinger's life.

This idea comes up from time to time. It's not impossible, but I find it rather unlikely. Vyman is pretty old and presumably has been serving at Riverrun for many years, so I can't see what LF could bribe him with to get him to murder his lord. Money? He would have left RR if he suddenly became rich. Lands? Titles? He'd have to wait until after the war, and this would look highly unusual.

While I'm loathe to chalk up positive outcomes for LF as pure luck, I make an exception in this case. Hoster was an old man and he died. It happens.

[The Commentator]

He was poisoning Catelyn’s mind.