I had the understanding it was called "reversion" where the engine tended to spit fuel out of the carb, normally there is a vapor cloud "standoff" occuring just at the mouth of the carb too. But with model airplane engines the vaport cloud gets blown away causing the spitting to appear. The carbs do not care which way the air is flowing, so the jets will flow fuel in either direction.
Two stroke engines tend to behave similar to the 4 stroke engines in that the valve overlap where both valves are open is similar to the ports all being open at the same time. The pressure pulse and or shockwave affect it in the same manner. Also as mentioned already the air/fuel mixture tends to bounce when the valve closes stopping the intake flow. That bounce is quite similar to the water hammer effect in the plumbing of large buildings and large water mains pipes too. The air/fuel mixture doesn't want to stop, but it has no choice, but in the process it tends to bounce back out due to inertia.
There is a good explanation of it here http://www.circletrack.com/enginetec...ake_reversion/
and here too
My Harley Davidson can really show a pronounced reversion effect from the odd firing arrangement of the V-Twin engine. When I go cruising up a long mountain road at slow speed, the carb can actually start dripping raw fuel out of the bottom of the air filter, from all the gas vapor condensing and collecting there in the standoff vapor cloud just outside the mouth of the carb.