Why Rotax 912 "burps the tank"?
The Rotax 912 engine uses a dry sump, and does not have a scavenger pump to return oil to the sump. Instead it uses pressure escaping past the piston rings to force the oil back into the sump.
The "burp" you hear coming from the (open) oil tank happens when most of the oil in the crankcase has been returned to the tank: the air escaping past the rings bubbles up through the oil tank.

The propeller is pulled through "forward" (in the normal direction of rotation) for a few reasons, mainly that it's the direction that the gearbox &amp; accessories are all designed to turn. Turning in the normal direction of rotation also ensures that oil is pumped through the rest of the system, particularly the valve train.
Reverse rotation can pump air into the system which may cause problems if you try to start/run the engine immediately after. I believe the recommended procedure is to "burp" the engine a few times to re-prime the oil system if it's pulled through backwards.