I don't know, but if you can look at logs and run a query for max IH shield, I'm guessing it happens more with old gear. Much more with periods that lack damage and EF rolling into IH.
I'd be curious to see some of my logs though, I'll have to look. I'll also have to remember not to count the ones where tell me when broke my raid frames and I started healing off of nameplates. It would be great if my frames/tmw didn't seem to randomly move once in a while.