Shot in the dark. 100°?Basic physics. The blade is very sharp, so the force to penetrate the sidewall is small.
The angle is steep~45 degrees, so the imparted vertical force is about the same as the horizontal force (which would knock it off the tripod).
The inertia from the mass of the water not wanting to move, plus the additional downward force from the blade, times the static friction coefficient is more than the force required to move the bottle so it doesn’t slide off.
Homework for the reader: at what angle of slice will the bottle start to move before the cut is made? (Hint: It could be slightly upward)
Thanks, stuff like this is why teaching physics was fun!