That's what we call a 'technical' mounting a recoilless rifle, and from the looks of it, I'd assume one of three things happened.
A) There was an obstruction in the back, causing the back-blast to be redirected and causing the explosion.
B) In the reverse, the barrel may have been obstructed, causing the full explosive force to be directed out the back.
C) They used a standard shell instead of a recoilless shell (Possible, and very very bad. Pay attention to your ammo, kids)
Upon closer examination, it looks like the gun itself is intact, but if you note the angle of the back vent, it's aimed down into the truck bed, which would have then received an explosive back-blast equal to that propelling the projectile... in effect, ripping the base of the truck to pieces.
This is why, when we use such weapons in the military, we make sure that the back-blast area is clear.