What you are seeing is typically called wing stall in aerodynamics. Stall is a condition of lift drop caused by flow separation at a wing. As the AoA increases, the flow starts separating from the top side of the wing. The flow initially separates near the trailing edge, then the separation front moves upstream as the angle of attack increases. Past the critical angle of attack, which is what you are seeing in the plot where the sharp decrease begins, the pressure distribution on the top surface of the wing starts increasing due to the large flow separation. The lift generated decreases meaning that the wing has started stalling. During stall, a small increase of the angle of attack produces a significant drop of lift generated by the wing.
I hope this answers your question.
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