IMHO the #1 reason is that the crankcase is loaded with excess fuel, resulting in a large fuel charge in the cylinder igniting prematurely - enough before TDC to kick back and continue running. This can be caused by more than one reason, such as overzealous priming or a too rich low end mixture. A fuel tank too high can cause gravity fuel feed into the spraybar thereby loading the case, or fuel plumbing/filling issues could also force excess fuel through the spraybar. Another possible cause is plug heat problems, i.e. where the filament is not getting enough current, allowing fuel to accumulate during cranking without firing, then firing after excess fuel has accumulated. In any event the result is that the case accumulates excess fuel.

With the excess fuel in the case, the issue of reverse rotation is somewhat negated by the readily available fuel and whatever air it manages to inhale. I don't know the mechanism of how the porting and all that behaves when it is spinning backwards. I'd say the issue is overpriming or other forcible entry of fuel into the crankcase when the problem is immediate, i.e. the first attempt to start either by hand or starter burp. I'll bet most people have experienced the starter kicking back at you as a wet engine took off backwards? If the problem surfaces after a false start or two, that is, where there has been an opportunity to get some fuel draw happening, I would then look at the idle mixture setting instead.

If the engine is really loaded up I generally disconnect both the fuel and vent line from the engine, check that the plug is up to a good bright orange, then bump it backwards (not through compression) with a chicken stick (oh yeah - this is definitely the time when the oft-printed advice to use a chicken stick should be heeded) - if it kicks off running in either direction, I let it go. It will burn off the fuel in a few seconds, and if it is running backwards (usually it will be since that was the problem in the first place) as the fuel dwindles they often hiccup and run forward briefly. I've been known to experimentally plug the fuel line back onto the carb just as the engine is dying - btw to suggest doing this is stupid advice but at least follow the reasoning here at least - and watch the response. If it keeps running and seems to be running smoothly on the normal to lean side, I then plug the pressure line back to the carb. If it still seems to run okay, then it was likely not mixture but fuel loading for a reason I would then have to determine. If however it progressively richened and died, then obviously I need to lean out the idle mixture. If this happens I usually repeat the fuel burn-off procedure before trying to continue, in other words to get a fresh start with a chance to analyze what's going on again.

Just my two cents. MJD