A team using Rossi x-ray satellite data (first link) found one neutron star that seems to have the long-expected behavior; and maybe a clue as to why that behavior might actually be rare. This neutron star spins at only about 11Hz, compared to the O(200-600Hz) that seems to be more typical. The story says one possibility is that there is friction between the fast-moving star surface and the in-falling plasma that could change the way the plasma heats up.
The plasma is swept towards the magnetic poles, resulting in large local currents whose own magnetic fields interact with the current and the existing field of the star to produce pinches and reconnections and similar chaotic effects. (Visualize the lively nature of the corona of our sun.) You expect the in-falling plasma to still have considerable angular momentum, so it should be whirling down to the poles very fast, perhaps rotating faster than the star itself. (In fact the star is expected to slowly speed up thanks to the gas from the accretion disk falling on it.)
The picture in my mind is of a tornado of plasma and magnetic fields; and I'd expect that while some energy will be radiated away in reconnection radiation (X-rays, highly accelerated particles, and the like), much of the rest will be imparted to the plasma long before it hits the surface: arriving already hot and not moving as fast as the naive calculation would expect. This should be a bigger deal for the slow-rotating star with a larger difference between its own rotation and the tornado rotation. Unfortunately I'm not sure whether pressure or temperature matters more for this kind of nuclear ignition, so I can't predict whether the fast-rotating star's faster impacts or the slow-rotating star's higher temperatures would cause more frequent bangs.
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