I was sifting through the MTW Guides and saw a number of references in the Russian guide when they talked about the appearance of the Horde. A number of posts were made about rebels attacking. Here is what I gleaned:

1. Rebels will attack although it is rare.

2. The Rebels typically severely outnumber the army in provence they are going to attack before they commit to an invasion.

3. It also seems this is more of a problem with certain provences on the Eastern side of the map although I can't remember which ones they are.

That part of the thread was started when someone mentioned he killed off the Khan and heir and they went rebel. The huge rebel stacks then started attacking outlying Provences.

Alot of the posters didn't seem too surprised and gave bits of info about there own problems with rebels attacking. The MTW Guide for the Russians/Novrogod is only 3 pages so its worth the read.

Maybe its just more surprising to us players who don't play the Eastern side of the map all that much? I know I typically don't play Poland, Hungary, and Novrogod, which are the Factions near the more aggresive Rebels.