mobile

My wife's nexus 5 seems to be shutting off randomly. Now I've had android long enough to know occasional random reboots are unavoidable but hers isn't rebooting it's just shutting down. Like this morning she went to get it off the charger and it was turned off. Also she'll pull it out of her pocket and it'll just be shut off. Hardware issue or known lollipop 5.1 issue? Seems to me random reboots are more common when it's software related. Never heard of a bad ROM causing a phone to shut down entirely