Its to my notice that if you only own a few provinces rebellion is highly unlikely (excluding civil war). I've always wondered how the heck the A.I was getting away with rebellions with such small garrisons until I was weak myself. The game is designed not to hurt the small guy but aims its intentions against the big guy.

Now when you get close to 60 percent I have had rebellions in regions with high loyalty. It is a bit of a game balance, also when I was the weak guy rebels would join me(out of sympothy I guess) but this help turned the odds in my favour (only to get slaughtered a few years later)

This happened to me in VI as I thought hey I'm against the A.I. again I'll just train mobs of peasants and make the A.I. think I'm strong while I am upgrading farmland. Yeah right I got mobbed like I was the A.I. The rebels kept joining me but to no ado as I was getting clobbered by superior troops