honestly, I don't think it can be anything else besides little micro implosions or explosions of combustible fumes, not necessarily the whole chamber even, that would be the big scary ones that make the cap jump, but most times just small bubbles of concentrated vapors, maybe a tiny void in the wick which would be like an air bubble, only a volatile fume air bubble, can't see anything else that would really make that popping tbh, the only other theory I could possibly come up with is juice boiling and then bubbles popping off the boiling juice like bubbles would pop in boiling water but I tend to think that much more often it's little gas pockets, boiling wouldn't pop as much, would maybe make it spit juice but not as much the popping I think