Vasalls only react to rebels after rebels win a siege

[K-vald]

Are you sure that the siege was actually an on-going one? Since the AI is more or less meant to mimic human behaviour (rather than being super good) I can see it plausible for it to wait. I usually wait to engage sieges too, especially if they are on hold, meaning the defending force are larger than the attacking one.

 

[K-vald]

This is a good example:
Rebellion in Gniezo. No one reacts. Despite having the Count of Gniezo, the Duke of Greater Poland and the King of Poland able to defeat them.
View attachment 49311

The day the first siege is lost, everyone suddendly reacts and raises his troops. Even the Count, who can now only raise his cities vasalls levy
View attachment 49312

Good screens, I will look into this since it really looks like the trigger doesn't go off until the siege is half lost.

 

[K-vald]

The OP is talking about a peasant rebellion. Don't those always go straight into siege mode?

They do, but there can also be a case where there's not enough peasants to actually outnumber the defending force, but since this wasn't the case (as shown in the screenshots) you may disregard what I said.