It could have a seriously large amount of valve timing overlap where the cams are so hot that the inlet has started to open when the exhaust has not yet even near closed. At high RPMs (while racing) the harmonic resonance of the exhaust pipe will provide a low pressure pulse to the exhaust port, but at lower revs the exhaust reversion pressure wave will arrive at the combustion chamber instead, pushing gasses back out the wrong way for a fraction of a second. Especially since the piston may still be moving up the cylinder bore at that moment. The pressure in the pot may well exceed normal atmospheric pressure a the intake valve under those conditions. But yeah, it's spitting a LOT out the carbies