Why the open mic all the time? There is a lot of it?
Probably a case of muscle memory. With aircraft depressurized the pilots are wearing oxygen masks. The only way for them to communicate is to use the crew intercom. The PTT button has two positions. Up to transmit over radio and down to communicate on the intercom. Muscle memory probably caused them to press up to talk to each other, and when they did, they transmitted and ATC recorded it.