How Does This Happen?

We've given up clocks in our bedroom. Both Richard and I use our cell phones as clocks and alarms. Here's Richard's phone:

Explain to me please how the alarm on this phone can go off at 5:00 this morning when it *didn't* go off the last two days. 5 am! Yeah, I never did go back to sleep.

But at least it wasn't some loud, annoying alarm sound. I was awakened to the sounds of the phone vibrating on the table followed by the soothing tones of Enya. If it hadn't been for the buzz of the vibration, it wouldn't have been quite so annoying. Well that and if it had been at, say, 8:00 and not 5:00.