Question Torqueflite A904

exactly what is occurring (" If you are accelerating rather quickly and then back off the accelerator, the transmission will quickly shift to third gear ") ..
it's normal?? What is the explanation for this happening?

Completely normal under the circumstances.
What happens is the trans pressure is higher with more throttle and when you start backing off the throttle the pressure goes lower and it shifts.
Sometimes with quite a driveline ringing slam.:D

Completely normal.