I’d guess the front landing gear assembly is going to be fairly heavy, and appears to be missing. This model of plane also has its engines at the rear, not under the wing, which will move the balance to the back.

It's balance, you don't need a large difference in weight for it to tilt backwards.

Planes typically have their center of gravity just forward of the rear wheels. This makes it easier to rotate on takeoff.

The margins are thin enough that certain planes will sometimes have people in the back get off first, before the people on the front, to avoid tipping onto the tail like this.