Thing is, while grappled you can choose not to break free but to attack (in a normal humanoid-on-humanoid encounter, a dagger to the guts of your opponent as they grapple you and move you to the cliff edge).
But you couldn't realistically use your longsword terribly effectively in this scenario, so your attack options are limited. Also, despite that knife in the guts, your opponent is still grappling you. You're banking on killing him before he drops you to your doom.
A shove is an alternative to a normal, damage-inducing attack. It presupposes two combatants squaring off against one another, where A chooses to shove B further away from him. It implies distance (melee range) which is widened. Neither A not B is grappled.
If A is grappled, a shove won't break a grapple. It will push B a certain distance away, but B, having not relinquished his grip, will just take A with him. Useful to stave off that cliff drop, sure, but not breaking the grapple.
Now, if the fighter breaks the grapple (twist that hand, sink that elbow in), then action surges to shove, sure. But a shove on its own wouldn't break the grapple.
Especially if you're not fighting mano a mano, but instead a croc has your leg. How would a shove even work? Meanwhile the croc could be ripping at your leg like a dog with a toy, knocking you onto your back and dragging your supine ass backwards into the water where it will then hold you under by its own weight, lying on top of you until you drown. In the dragging to the water part, you have nothing to shove at, the best you could do is kick it with your free leg to either damage it (normal attack) or dislodge its jaws (attempt to break grapple).
edit: video here showing how you end up prone (arm not leg to be fair), but a shove ain't gonna cut it either way
http://youtu.be/6ZhHHVsAnI4