watches unpressurized

A question to the group. When I was going through my high-altitude
physiological training, they instructed us to remove wrist watches
before going for the baro-flight to FL250 (unpressurized). Has
anyone experienced any anomalies with wrist watches at low pressure?
I just can't think of why they'd require that. Even diver's watches
will hold the pressure up to a point and then the seals will leak
-- before any damage to the body of the watch or the crystal occurs.
Does the battery have any air inside that will blow it up?


Andrey