My experience has been that the thumb on top grip leads to the "beginner" floppy back cast. The thumb on top grip aligns the fly rod across the palm at almost 90 degrees to the forearm.
Unless the caster locks
the wrist cocked forward, the rod tips way back as in the photo below. For beginners, this is an unnatural position to hold the wrist so the thumb on top grip does not prevent
a floppy backcast.
Gary Borger » Blog Archive » Thumb on Top Grip
The three point grip (white line below) aligns the fly rod closer to the forearm. So at the back cast stop, the caster does not need to lock the wrist cocked forward to "correct" to prevent the rod from tipping back too far. The three point grip naturally places stop with the rod tip at 1 o'clock.
I change beginners from the thumb on top to the 3 point grips if they continue to have problems.
Gary Borger » Blog Archive » Three Point Grip Part I