I've never heard of that happening. You're going to need to try to determine how it happened, I guess. What do mean by "pops off"? Stripped threads? Did it come off the bracket? Did the bracket bolt snap?
Pics I'll post in morning but here's the deal, I hear a weird whine from the engine bay sounds abnormal obviously so when I get home, I see my 90mm aux idler pulley is missing
Because it's gone, my belt is literally now rubbing on the side of the metco aluminum aux aftermarket block... Hence the whining sound
And... I look down and my 90mm pulley is resting between the radiator and one of those small support brakets... Literally down near the bottom of the radiator
I look at it and both bearings are still in it... The cap and bolt that hold this onto the aux bracket are no where to be found
Yeah the bolt backed out. Need to use some Loctite. You will probably need to get a new mount as it looks like the belt "sanded" one side flat. Don't think the pulley will roll correctly on that surface now.
Call Metco. Every time I have worked with them they have bent over backwards to help me.