Ah, interesting encounter. Were the shambling mounds summoned or created? The diifficulty of six of them is such that I believe there is a reason they were made "solitary" creatures heh. Rather, those kind of encounters are the "run-away" and tell those fireside stories about the "big-ones" that got away, lol.
The grapple checks, as I said, it shouldn't be implied that the winner gets to do what they want. Who initiates the grapple check is important. In the grapple section (PHB): "You can make an opposed grapple check as an attack. If you win, you can do the following..."
If you fail the opposite occurs, you can't do the following... that's it.
A grapple check is essentially an attack. Two combatants are wrestling, one tries to do something (choke, escape, dagger stab, pin, etc.), the other opposes. If they succeed they can do it, if they failed then their opponent stopped them in their struggle.
Incidentally I neglected the fact that shambling mounds get the size modifier to grapple checks so a shambling mound has a +14 grapple check for being large. No worries, 3.5 will have a stat line for grapple modifiers in the new MM. Nasty beasties, shambling mounds.
