While playing in Adventure mode while on my multi-player server, I encountered a single zombie in a house during the day. I retreated and the zombie chased me out into the sun, catching fire. Soon afterwards, zombies began to rain down from above, increasing the number of opponents dramatically.

Only once I moved a significant distance from the nearest burning zombie did the zombie rain end.

I have this on video, as I was recording a Let's Play when it happened.