I agree with John that the wing must have lifted which increased the incidence to effect the sudden pop up of the airplane. Same thing happen to me over 40 years ago with a Falcon 56.
I have never been a fan of the PT-40 trainer. It was designed to fly with absolutely no pilot input, and about anything the pilot does just upsets it's free flight tendencies. The amount of decalage and it's balance point means it will fly level at just one speed. Any change of power will cause it to climb or lose altitude much more than what is typical of most trainers. Much of this can be taken out of the design with changes to the wing incidence and engine thrust. Some variations of the PT-40 also have excessive dihedral, though later versions of the kit showed a lower angle to be used.
The original design would have been well suited to fly with just rudder and throttle control, never touching the elevator or ailerons.