Roger,

I had this same problem a while back with my bath fan. The problem was
a minor leak from the roof vent. Fixed leak. Haven't had this problem
since.

My in-laws just had a new roof installed, so I didn't want to cut any holes
and risk causing a leak. So we routed the duct out through a sidewall.
Except for a short bump up over a beam, the 8' duct slopes mostly towards
the outside wall vent.

So, there's no chance of a leak, other than maybe condensation working it's
way back. However, the tripping doesn't seem to be related to shower usage,
which makes me doubtful of a moisture related cause.

Anthony