I doesn't currently just come off a to-hit roll, it requires and opposed roll.
"Instead of making an
Attack roll, you make a
Strength (Athletics) check contested by the target’s
Strength (Athletics) or
Dexterity (Acrobatics) check (the target chooses the ability to use). If you win the contest, you either knock the target prone or push it 5 feet away from you."
So right now if you're good at Grappling you're good at resisting a Shove, which means it usually fails.
With the new rules, it'll be much easier to Shove people away in virtually all cases.