I think you were asking this question related to wing physics and not to airfoil physics. Here comes the physics of induced drag. I will try to simplify the explanation.
As you might know, the upper surface of the wing has lower pressure and the lower part of the wing has higher pressure. This is what is producing lift.
But... what about the tip of wing? Both sides are in contact so they should be at the same pressure?
So, we have a lower surface of the wing with higher pressure and the tip with "normal pressure", so, naturally in the lower surface the air will try to go from the root to the tip.
On the upper surface the situation is the opposity, the root will have lower pressure and the tip higher, so the air will try to go from the tip to the root.
As in the tip both sides are connected, the air will try to scape from the lower to the upper level (that's why there are wing tips). And that's why the tip vortexes are created with the typical pictures
Ok, now let's talk about drag. Drag is actually "any energy we are losing just because we fly" (ok, is a simplification but allows me to explain the next step). So, the airplane is losing energy and gives it to the air to create the tip vortex. The bigger the tip vortex the more energy is lost.
So, know coming to the question, why efficiency (L/D) decreases? So, as the angle of attack increases the lift increases linearly (proportional to the increase of the angle), however, the energy lost in the tip vortex (that is a component of drag) increases quadratically (proportional to the square of the angle of attack), that means that the drag is increasing more quickly (but starting at a lower level) that lift.
(lift grows, but drag grows proportionally faster)
So, we reach a point where the increase of drag makes the efficiency starting to decrease. That point can be calculated with the formula provided in the previous post.
Notice that the physics of the wing is more complicated as there are other drag components also interacting with the wing. It is also true that "infinite" wings have a maximum L/D, but there is a different mechanism in place.