what could cause a sudden drop in battery life?
It could be a misbehaving app keeping the device awake; I have that about once a month with my Motorola Milestone 2. Did not figure out how to prevent it, but a reboot always solved it.

A proposal how to "catch" it, however: There's an app called DrainGuard available on the playstore. It watches your battery consumption, and gives an alert as soon as it detects a "sudden drop". This helps to notice the "power drain" before the "5% alert" makes a noise and switches your phone off -- so you can either start investigating while the process is ongoing (some time left before "full drain"), or simply trigger an immediate reboot to "kick the bad guy out" when in a hurry.