Why?

I don't know. None of the books explain it. They just descibe
methods of correcting for it. (Test pilots seem the practical sort,
rather than theoretical.)

My suspicion is that it's due to the "dynamic stall" concept. When an
aircraft is rotated rapidly to a high AOA, it can generate a much
higher lift coefficient than steady state. Apparently it takes some
finite amount of time for the adverse pressure gradients to do their
magic and cause the airflow to separate.