At first look I thought it might be related to still having too much moisture, and the cells rupturing out from the side, though I don't think it was a major cause(though it still could factor in somewhere in the problem).
Definitely an unusual crack. I think there are a lot of factors involved. That clamp is right above a knot, which turns it into a real stress point, when you crank down on the next clamp in sequence. Kind of like using your knee as a fulcrum when breaking a branch for firewood.
Combine that with not enough time with heating to penetrate right into the centre of the stave.
Easy enough thing to do, especially if you haven't done many staves.
I found since using cooking oil to heat in a reflex or straighten I have had less problems. Somehow seems to help the heat penetrate deeper, or trap it in better.
Also wait for the wood to relax. If you tighten the clamp and still get resistance at that spot you need to heat it for longer.
Like the other guys have said super glue and clamp. Just make sure to pad the back with a thick piece of leather so you don't bruise the back with a clamp. Don't reheat the wood as the superglue will vaporize, and the crack will open up again.
