Deep fry volcano - anyone know why?

Thanks, ChefOno. This is great information just on general principles. I'm pretty sure I didn't overheat the oil at any point, definitely not to smoking. But you never get really pristine oil, once it's been used, do you? I'm betting on the coconut oil, maybe the mix of oils. Whatever, I'm not going to mix coconut oils with any others ever again. ;) What a mess that was.

I'm not sure I can answer the question from the information given but here are some things to consider:

Ferocious bubbling is the usual reaction when water vaporizes in hot oil, but for bubbles to turn into foam requires some help. Particles either dissolved or suspended in the oil can create a foam (first clue) but the foam will quickly dissipate unless there's something present to stabilize it. That could be lecithin from egg yolks (in your batter or from a previous use) or…

Used oil contains soaps formed by contact with food during prior sessions. That may sound yucky (who wants to eat soap, right?) but it's actually a good thing. The soap helps oil and water "connect" in the same way as when you wash your hands but in this case it improves heat transfer and thus browning. This is why food fried in used oil browns so much better than in new oil. If you've ever wondered why your french fries sometimes don't brown, they're a real b**** to brown in new oil.

However, when oil breaks down, either from being heated to its smoke point or from repeated heatings, it forms free radicals. There's a balancing act required between the benefits of used oil and from taking it too far (or for too many times).

Oops, I messed up. Pierino, the temp was fine -- I always use a thermometer. Would there have been an effect from moisture in the fish with the double coating? I patted the fish dry first, too, and I would expect spitting maybe but not froth. There was no burning or unpleasant smell, just a huge amount of foam.
Also, now that I think about it, it was extra virgin coconut oil, from a small supplier, and I'm wondering if there may have been bits of coconut stuff in it that could have caused the foaming. As I said, I've only used it to fry on its own prior to this.
Thank you

Was the fish still frozen or partially frozen? It's the only reason I can think of that would produce an oil volcano.