Curious Nerub’ar Palace Questions…

So, last night in Nerub’ar Palace (Normal), just after we pulled Rasha’nan, my priest was changed into a spider, and could not cast spells. The effect lasted ~15secs, which I think is excessive for it to have been some mechanic of the boss. I have not reviewed a combat log (we ended up calling a wipe to reset) to see if some in the raid was pulling a prank by using some toy on me.

Anyone else have this happen to them? Or know what a likely culprit it would have been?

I can say that I did not use Arachoserum toy on myself, nor eat anything that would have caused the effect.