I recently had cracks like that pop on a small scrap wood osage bow I was farting around with. It happened when I put tension on the belly attempting to add recurve before I began heating. The limb was thin enough I should have been able to bend it into a circle normally, but it cracked just as the surface began to heat.
My theory as to why this happened is that by bending the wood THEN adding heat the very surface was hot but the underlying wood wasn't and the wood just couldn't take the difference. ALWAYS heat before attempting to bend recurves - that's the lesson I took away.
BTW, I grew up west of Minot. Well, that's a lie. I got bigger. I have yet to grow up. Lemme know if you are ever coming through the Black Hills. I can act as tour guide, fishing guide, or just hang out in the shop and futz around with wood/sinew/rawhide/leather/etc.