LIke Hawk1975 I've been seeing quite a few factions with most if not all of their vassals sitting in towns / castles for months while they are attacked (seen it most recently with Mamluks, Granada, Novogrod, Golden Horde, al Hafsi and English). When you ask "What is the realm doing" they answer "[Somewhere] is under attack and we must ride to its defense". When you ask "What are you doing" they answer "I don't have enough men so I need to raise them before I do anything else...". Point is all of them have normal troop numbers (100+).
(I've checked and they aren't gathering for feasts / tournaments).
Edit: I'm on rev 46