If the random point is too far away from the Demolisher or unreachable then it'll probably give up and begin autoattacking. Also, it's likely that the AI is giving them direct orders, which you could fix by changing their Player Property from Computer to None. You can temporarily change their ownership during the cinematic if you eventually need them to be Computer-owned again. Worst case you could try to brute force their Attack Ground order by issuing new orders on a fast timer (ie: every 0.10s), that way even if the AI overwrites their order it will almost immediately get replaced with what you want.