Was there a catalyst for Roose Bolton's betrayal?

[The Academic Wolf]

And Rodrik didn't know it was the ironborn. That's my point.

Who else could it be? This is may simply be the way that I read it, but the impression that I get from reading the exchange about raiders being on the Stoney Shore is that everyone knows it is the Ironborn without having to come right out and say it.

[Khal BlackfyreO]

Catalyst = Opportunity

[Lord Martin]

And Rodrik didn't know it was the ironborn. That's my point.

he knew:

From Theon:

"To strike terror into the heart of the foe, as only one of your name could do. You'll take the great part of our force and march on Torrhen's Square. Helman Tallhart took his best men south, and Benfred died here with their sons. His uncle Leobald will remain, with some small garrison." If I had been able to question Benfred, I would know just how small. "Make no secret of your approach. Sing all the brave songs you like. I want them to close their gates."

...

"Leobald will not know that. When he sees you raising siege towers, his old woman's blood will run cold, and he will bleat for help. Stay your archers, Uncle, and let the raven fly. The castellan at Winterfell is a brave man, but age has stiffened his wits as well as his limbs. When he learns that one of his king's bannermen is under attack by the fearsome Dagmer Cleftjaw, he will summon his strength and ride to Tallhart's aid. It is his duty. Ser Rodrik is nothing if not dutiful."

and from Bran:

Torrhen's Square was under attack by some monstrous war chief named Dagmer Cleftjaw. Old Nan said he couldn't be killed, that once a foe had cut his head in two with an axe, but Dagmer was so fierce he'd just pushed the two halves back together and held them until they healed up. Could Dagmer have won? Torrhen's Square was many days from Winterfell, yet still . . .

[The Fallen]

Who else could it be? This is may simply be the way that I read it, but the impression that I get from reading the exchange about raiders being on the Stoney Shore is that everyone knows it is the Ironborn without having to come right out and say it.

When Bran has dinner with Rodrik and they discuss the capture of Reek Maester Luwin and Rodrik have this exchange:

Maester Luwin tugged at his chain collar. "The Reed boy believes he sees the future in his dreams, Ser Rodrik. I've spoken to Bran about the uncertainty of such prophecies, but if truth be told, there is trouble along the Stony Shore. Raiders in longships, plundering fishing villages. Raping and burning. Leobald Tallhart has sent his nephew Benfred to deal with them, but I expect they'll take to their ships and flee at the first sight of armed men."

"Aye, and strike somewhere else. The Others take all such cowards. They would never dare, no more than the Bastard of Bolton, if our main strength were not a thousand leagues south." Ser Rodrik looked at Bran. "What else did the lad tell you?"

It doesn't seem clear that they believe it's the ironborn. Maybe Martin omits it on purpose to fit the story line better? I also got the feeling that if he did know, they thought it was just some ironborn raiders, not an invasion.

he knew:

From Theon:

and from Bran:

This is from the chapter following the one I quoted above. It seems that he knew. As I stated above, it seems that Rodrik's actions were necessary for the story. But it's not bad storytelling in that Theon is never mentioned. We don't know if Robb had informed Rodrik of his sending Theon to treat with his father. It's a possibility. And Winterfell is so far away from the North's western coast that I don't feel they could have expected an attack.

Leaving a token force at Winterfell during wartime is inexcusable though. Eh, like Theon said in your quote, maybe old age did stiffen Rodrik's wits.