This is a problem I've had assaulting Syrakousai, too. It seems to only happen to large stone walls, though, because assaulting the ordinary stone walls of Roma didn't seem to cause any siege-tower distress. It took ten minutes to get my Gaesatae up the walls. It was a good thing I didn't place the tower anywhere within two towers of the enemy, so they didn't march over to see what the antics were all about.

Incidentally, another problem. When fighting the wall garrison with a friendly tower at my back, the tower fires into the fighting crowd and coincidentally does way more damage to my own troops than the enemy. It does not particularly help either when they are Gaesatae doing the assaulting... I was fighting a full-size unit of Poeni militia and they shouldn't even have killed more than 2 Gaesatae, but when I had finished with them I was reduced to 90 Gaesatae from a fullsize unit of 120. That tower (captured by me no less) did more damage to me than the entire garrison of the city did (3 gaesatae killed). It was ridiculous...