How to prevent Loaf Burstage during baking?

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I recently baked multiple loaves using Bob's Red Mill Artisan Flour. I am using a pizza stone to bake my loaves. Once the oven is preheatedto 250 c I insert the dough and immediately add boiling water to the bowl of lava rocks right beneath the stone for steam. I have noticed 10 minutes in the loaf transforms quickly and there is an upward bursting from the center. For anyone asking I use 20% sourdough starter for every 1 kilogram of flour and my hydration is 75%.

Why is the loaf bursting? I can't tell if it's underproofed - I have cold proofed for 24+ hours in the fridge plus 2 hours on the counter before baking. I also did the poke test it seemed ready to bake. My hunch is I should let it proof longer + 2 hours more. Can anyone help?

 

Thanks

It's blowing out in one portion of the loaf (not the top).

Shaping error?

Uneven heat in the oven?

Steam underneath the stone cooling a portion of the stone more than the other side?

THanks. It seems to me that it is a mix of underproofing and crust drying out.

The final proofing duration was extended for 6+ hours. There are still some signs of burstage but not as much before. I also changed the position of the bowl unerneath the pizza stone to which I add hot water for steaming - it is now placed exactly under the pizza stone.

This more than likely set your crust or dried it out.  Try a longer bulk ferment and a shorter cold proof being careful to keep the crust from drying too much.