Its kind of hard to describe. You can tell something isn't right and the most obvious thing is you'll ride up, do a swing. Maybe there wont be any audio with it. then nothing will happen for a few seconds and you'll wonder if its your turn... then he swings. Or sometimes theres no swing at all and no sound and the enemy unit just flies back and dies.

Since the only thing the mounts do is +1 to moves and combat moves and the shortsword gives +1 combat moves I think its something to do with that combo.

I was playing as pariden at the time.