Interesting problem jr. Like hashie, I have never heard of this one before* so I can only guess.
I imagine that your burner is quite powerful and it is boiling the liquid between the false bottom and the kettle base and this hot liquid (and bubbles) are finding the easiest way up and out - the sides.
Make sure you don't use a traditional mash paddle. Use the potato masher type one as pictured in The Commentary. These move the grain much better.
If you are step mashing or mashing out, get yourself a rope and pulley as this will save you agitating the grain every time you apply heat. Drop your paddle into the bag and then lift it so it is of the bottom. Give it a jiggle every few minutes to even the heat out and do this until you reach your desired temp.
Good luck,
PP
* I do actually have a dim memory of getting the occasional bubble up the side in my early days of BIAB when I was a bit lazy stirring. You could actually hear the bubbles forming at the bottom of the pot and this was without a false bottom. Ah, yes, it is all coming back to me now
. After a couple of guys burnt their bags by not stirring, I lifted my stirring game. About 3 years later I finally moved to an apartment where I could use a rope and pulley.
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