I am not able to test this and it is nearly impossible to provide SS for it since I was d/c'ed when it happened. Note: this is not completely a theory since it happened just as I d/c'ed.
Earlier today I did Lord Rhyolith normal (could work in heroic) with the guild in a 25 man raid. Wow froze on me (it does this occasionally) while I was aoe'ing with blade fury (yes, I'm a rogue and was told to have the boss go straight) and had threat on one mob. I think it was a Fragment of Rhyolith - NPC - World of Warcraft, not 100% sure. Not 100% sure if it was more than 1 add either. In any case, when wow froze on me, I closed the window and opened up a new process to login like normal and I heard alot of chatter on vent saying the adds de-spawned and for everyone to push the boss into phase 2.
I find it very odd that my d/c coincided with the adds despawning and am wondering if someone can help verify by reloging the game while having threat on at least one add to see if d/c'ing really is what is causing the game to de-spawn the adds. I'm guessing that you have to disappear from the actual game world, so you will have to have someone in raid to check if you actually go offline or just stand around running in place doing nothing.