Ran into a situation where some units are fighting through each other's engagements.
Cavalry is fighting irregular foot across the superior warband/spearmen engagement.
Unfortunately I don't have a save handy. The superior warband and the cavalry were both pursuing a third unit and both engaged "simultaneously" into their respective targets. Don't remember who engaged first.
I guess this is the result of the somewhat chaotic situations that can occur as a result of multiple "simultaneous" pursuits.
The game calculates quite a lot of stuff before it is seen to happen on the screen, the events are queued to be shown. Otherwise it would be impossible to view the action in a comprehensible way. The units aren't actually moved until their action reaches the top of the queue.
In this particular case, therefore, it is likely that at the time the two charges were put into the queue, the other units weren't in combat, so the normal checks that prevent units from charging diagonally across another combat did not work.
Perhaps this could be prevented, but the code required to do so would be very complex, and might in turn cause other issues. These sorts of anomalies don't happen very often, and it probably isn't worth the risk of possibly breaking something else in an attempt to "fix" this particular issue.
Just put it down to the chaos of battle. It doesn't mean that the game will allow diagonal charges across a combat in normal cirucmstances - it won't.
