Why did the TIE fighter that Poe and Finn crash in get pulled into the sand? And after it was pulled in, why did it explode? Was there some sort of big creature like the Sarlacc that pulled it in? There does not seem to be any logical reason for this to happen.
The TIE-fighter was dragged down into the "Sinking Fields" that Rey mentioned to BB-8 a few minutes earlier. The ultimate cause of the explosion isn't clear (ammo vs fuel) but the proximate cause was a fire that resulted from the crash landing.
The film's script offers some direction.
REY [to BB-8]: Keep away from the Sinking Fields in the north, you'll drown in the sand.
[Finn scrambles over the massive dune to the TIE FIGHTER, SMOKING AND ON FIRE, small debris around it. He thinks he sees POE'S ARM -- he pulls on it, but it's JUST POE'S JACKET. He throws that off and tries to find a way in -- even a way to LOOK INSIDE, but the smoke and heat make it impossible.]
FINN: Poe!!! POE!
[Suddenly the ship BEGINS TO SINK -- like it's being CONSUMED INTO QUICKSAND -- Finn is slipping in too --]
FINN: POE!!! POE!!!
[It becomes clear fast: if Finn doesn't get away, he's gonna get sucked in too! So Finn scampers away from the TIE FIGHTER as it SINKS INTO THE SAND! We're in the Sinking Fields. A few moments and IT'S GONE.]
[Finn's out of breath. Horrified. Then A GIANT EXPLOSION ERUPTS FROM below, SENDING SAND AND DEBRIS EVERYWHERE!]
First of all, there are TWO versions of that scene: the standard cinema version (SCV) and the IMAX version (IMV).
In the SCV, there is no burping sound, only the explosion of the TIE after it's swallowed by what we could infer, based on Rey's earlier warning to BB-8, to be the Sinking Fields.
In the IMV, there IS a burp as the TIE explodes after being swallowed. This version is also the version you receive when you redeem the digital copy code in the Blu-ray + DVD + Digital packaging.
Now, if anyone has seen the FORCES OF DESTINY cartoon shorts, there is a logical explanation for the burp.
In two of the shorts featuring Rey and BB-8 on Jakku, there is a creature that lives below the sand whose diet consists of metal. This creature expands on a creature first seen in THE FORCE AWAKENS.
As BB-8 rolls away from the village of Tuanul, stormtroopers fire on Poe's X-Wing causing it to explode. The explosion startles BB-8 who stops, turns to see the fireball and continue on his way. As he proceeds, a creature with glowing red eyes pokes its head out of the sand.
This creature COULD be the one who burps after, literally, swallowing the TIE.
And you thought George was the only one who tinkers with STAR WARS films.
In sandy deserts, pockets of very fine dust can develop. If they are exposed they are dangerous because you can step in them thinking they are just normal sand and you will sink to the bottom and suffocate from inhaling the dust. The dust pits can also become covered by normal sand, making it possible to walk over the dust pit without realizing it is there unless you break through the sand layer. In this case the fighter apparently crashed above a large dust pocket and disturbed the layer of sand enough for the heavy metal to sink. The initial gouts of sand were from the displaced air in the dust pocket being forced to the surface around the sinking hull. When the burning fuel tanks were contained under the layer of sand it reached an explosive mixture of fuel/air causing the final blast.
The ship is obviously leaking fuel. As the ship sinks into the sand it is compressing the fuel. Together with the heat of the explosion, the ship explodes under the pressure of the sands of the SINKING FIELDS (that Rey mentioned to BB-8 earlier). It is pretty much the same concept as how a diesel engine works.