My theory is that when you camp in the small space of a teardrop and don't open the vent (like in the winter, for instance), the condensation that occurs on the dome can run into that pop up switch and futz it up...
Anyway, checking around on the net...corrosion on the pop up switch seems like a relatively common occurance and should be an easy fix. Reports are that the Fantastic Fan customer service department is excellent...we'll see!