Why is air pump making high pitch when it is overloaded?
This question really belongs in physics, but I'll take a stab at it.

Increasing back pressure on a positive-displacement air pump does not increase the load on the motor; it actually decreases it. On each stroke of the pump, less air escapes through the exhaust valve, and more of it remains in the chamber to help push the piston (or diaphram) back down, returning energy to the motor.

In other words, the output airflow drops off faster than the inverse of the back pressure, and the total work (airflow &times; pressure) drops.