The X-Bow shoots at its target, and it might kill it. The mortor also locks onto a target (which might happen to be the same one). Just as the mortor is going to shoot, the X-Bow kills it, so the mortor locks onto a new target and tries to shoot again. If the X-Bow and mortor keep locking onto the same target, then this will keep happening. This means that the towers are NOT just doing nothing. (I believe that's what Flammy said in one of his videos...)

