http://www.youtube.com/watch?v=31VpcWV2R9E&NR=1&feature=fvwp

2:17

I really don't know how anyone missed this. After all the cultists exploded, a pulsing white sphere appeared which seemed to shrink into oblivion, but then burst again. None of the monsters looked like bosses, so I conclude this is some rune effect of Exploding Palm. Perhaps when 4 or more enemies explode at once it leaves behind that effect, doing even more damage.

I should really stop finding these. I'm supposed to be working on finals.

EDIT: After examining more closely it is probably a rune of Way of 100 Fists. The dash was the last move the monk used, and the color scheme matches 100Fists more than Palm. But that also raises the question of why the cultist on the Top Left exploded without having the debuff? Perhaps that's another rune effect. Percent chance that anything killed by the explosion causes another explosion.

You can actually see that one of the cultists has a lightning effect around their feet (it's faint), indicating it's a champion or rare with an electrical affix. The effect you're pointing out is a post-death lightning explosion from that rare/champion. The same effect happens with all of the damage types if they roll that affix.

Rares/champions also have a faint glow to them that's actually quite noticeable in-game (that indicates they're not a normal enemy) but it doesn't really come through in the quality of that video.