Why topside torpedoes are a danger to their own ship

Did deck-mounted torpedoes commonly endanger the ships carrying them?

Not necessarily commonly, but they were a serious potential hazard if hit, much like other exposed explosive stores.

Topside torpedoes resemble depth charges and ammunition in one important respect: the risk lies in what happens when exposed explosives are hit. A torpedo strike or fire can turn a deck load into a major vulnerability. A commander therefore prefers to launch them when tactically useful instead of retaining the hazard indefinitely.