[thejeff]

It should not happen with the A->B and C->A scenario.

The A army should move into B and fight, while the C army moves into A and fights whatever was left behind.

The only way the A army won't move is the head-on scenario Niarg describes, or if A's commanders are killed before the movement phase, by assassins or ritual spells. (Any troops led by killed commanders will stay put.)

Is it possible troops from B are joining in the attack from C? That could put you into the head-on situation.

There has been discussion of a possible bug preventing movement from certain provinces, but it doesn't seem to be related to battles.