Siege defence bug

[Deus ret.]

Now this is something quite weird...

I finally got round to purchasing BI (had a gaming break for the last couple of months) and then, amidst my first campaign, I experienced something strange:
I was defending Londinium against the Romano-British with the Saxons, and they had built a considerable amount of siege equipment when they assaulted. they had, among other things, two towers of which one approached the walls under heavy fire from my two archers. now, they keep firing at the tower, my own wall towers help....and what happens? the siege tower deployed on another wall section begins to burn and then collapses without having taken more than, say, five salvos from a wall tower. Of course I continued shooting at the remaining tower....with the effect that one of the rams started to burn and was destroyed without having really being shot at. what the heck?? has anyone had similar experiences? those damned legionnaires in the tower I meant to destroy managed to get on the wall and beat the shit out of my wall troops (didn't have axes or hearths with me).
another weird thing is that siege equipment generally is displayed as being 0% damaged until a certain critical point when damage suddenly jumps up to 60% and then keeps on augmenting quite fast if the shelling continues.....the shelling of another tower in this case!
any observations, anyone? has BI introduced some kind of "overall siege equipment hit points" which causes random devices to be destroyed once the number passes certain marks? it's no issue with the installation because I installed it again after the incident, replayed the battle and it was all the same.... pretty annoying, that! my system is largely bug-free and BI patched to v1.6, so...I'm clueless.