Two ways -- first, as already a couple other people mentioned, the battery could get crushed in a hinge; and second, a phone which is turned on and wedged betwe

Two ways -- first, as already a couple other people mentioned, the battery could get crushed in a hinge; and second, a phone which is turned on and wedged between two cushions might overheat. This second danger is why modern phones are required to detect overheating and preemptively shut down -- but it's going to be decades before the FAA will want to assume that everyone has phones with that safety mechanism built in.