This is all just layman musings, but could it be due to the stimulation/inhibition mixture? The ramp-up to orgasm is marked by physical and mental stimulation mechanisms, but at the point of orgasm a neurological inhibition mechanism acts. It could be that by shorting the physical stimulation mechanism on your own, you trip up the inhibition mechanism.
Again, from the lay view, orgasm in the male is a product of 5 mechanisms or effects:
All these things have peaks over the time of the sexual event, and altering the timing of the peaks can deliver interesting results.
For myself, I've experienced effect #4 before, during and after effect #3. I found that singularly curious that ejaculation not only doesn't have to coincide with the actual orgasmic pleasure, but it can vary according to some still-unknown-to-me recipe of environment and activity. Don't get me wrong about that; events #3 and #4 are closely related and always overlap. However, they can vary by a few seconds in either direction. Naturally, one asks why such variance exists and how it can be forced to occur.