That worried me slightly too, although thinking about it realistically if the soldier was unloaded the horse would hit him, push him back and die from bayonet, which is basically what happened.

Unless Jack was obviously better in the line infantry I am unsure why the cavalry charged the squares, it can be seen as a daft AI move (rather than wait for the Polish line infantry to engage the square) or a clever AI move (last role of the dice/act of desperation knowing they'll lose anyway and doing something to try and force a win).

Also glad I'm not the only one that cheers when I win!!