The most common cause for that sort of thing with a phonostage is a massive static electricity build up that is being discharged through the phonostage. If the phonostage has some kind of protective circuit, it could be triggered by that kind of event.
Hopefully, you have some means to ground the main bearing of the table and the tonearm to alleviate that kind of static charge build up. However, I've seen systems prone to staic buildup, usually in the winter when the air is dry, even when there were provisions for grounding the arm and the main bearing. In that case, one will have to find a way to keep the air more humid to reduce such build up. Static discharge can send a huge wallop through a system that is, at its mildest, a big annoyance, and at its worse, a threat to speaker drivers.
Hopefully, you have some means to ground the main bearing of the table and the tonearm to alleviate that kind of static charge build up. However, I've seen systems prone to staic buildup, usually in the winter when the air is dry, even when there were provisions for grounding the arm and the main bearing. In that case, one will have to find a way to keep the air more humid to reduce such build up. Static discharge can send a huge wallop through a system that is, at its mildest, a big annoyance, and at its worse, a threat to speaker drivers.