Just got smoked in a combat that I *thought* wouldn't happen--was this a bug?

[geo981010]

UDH-
If you tried to move into an enemy province while an enemy tried to move from that province to where you were, the armies might collide such that one army gets pushed back and they battle at one of the two provinces. The chance of colliding is relative to the size of the two armies, and the smaller army is more likely to get pushed back.

And in my experince, you seem to be more likely to get pushed back in cases when you are sieging like that, but that might be just some random weirdness for me...