What causes standard door hinges to close up to a certain amount automatically?
The two hinges are not exactly one above each other, in 3D. One of them is slightly left, right, front or back, relative to the other.

As such, the line going through both hinges forms a vector in space. If this vector is not straight up, when the door rotates around it, the door will go higher or lower.

Gravity is pulling the door down. As such, it will try to go to the position where the door is lower.

(following comments, I assumed passive hinges. Do you have a hinge that is supposed to close automatically?)