UKPassats

Had an interesting experience with my thermostat this week. Temperature gauge suddenly dropped to zero, and wouldn't move. I thought it was the coolant temperature sender again, changed it no change. Then I decided to check the thermostat itself, and discovered that the pin that connects the thermostat valve to the housing had popped out of the hole, pushed it back into position, and reinstalled, and it works again, anyone have any ideas why this would happen? Thermostat is only a couple of months old.