Because applying DC to these piezoelectric devices causes electromigration, and therefore the filter destruction. Manufacturers warns about applying DC to the filters, but manufacturers have been ignoring this for decades.
A transceiver with this problem can be easily repaired by just replacing all the damaged filters, and to prevent this problem to appear again, a pair of DC blocking capacitors must be inserted at the filter input/output pins. In fact some manufacturers have modified their PCB in latest revisions to add these capacitors. But are these capacitors enough to keep DC out of the filters?
