Abrupt battery drops?

I usually charge my Zenfone 2 (4gb, 64 version) before I get below 25%, but yesterday I let in run down below 10 (I was seeing how much SOT I could get). My battery meter dropped from 10% to 8, to 4, then to zero in about 20 seconds. It happened so fast that the phone shut down before I could plug it in, even though the charger was sitting right next to me.

Anyone else experience this? Any idea what happened?