That’s a feature in literally all the Smash games tho. If you do an attack that does pushback on shields and the shielder in on the ledge, they will fall off. That is a part of the programming of the game itself and is also a competitive strategy for some characters. This is a feature that is also present in Melee through Smash 4, PM included. If your people are complaining about a mechanic like this, then they are just extremely out of touch with how Smash works in general.