Gravity. The aircraft is heavier at the back, where the engines are. With the nose severely damaged/missing, the centre of gravity has shifted aft, so what’s left of the nose is sticking up in the air.

Front fell off, people deplaned (while still horizontal) which shifted the balance backwards. It’s sitting on the rear bulkhead,

I guess there is more weight in the relatively small section of the front that came off than I expected

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.

I have been on small aircraft where some of the larger passengers (me, at about 100kg, and a couple of other guys my size) were asked to sit a little further back for the takeoff but move up front and get off last because of the difference in balance as the fuel was used.

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.

Oh yeah that definitely makes sense

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