Technical difficulties.

[tries to eat all of the bunny bouquet at once] Lemon balm, why aren't you in my mouth?!

That wayward sprig of lemon balm was definitely slowing down his annihilation of the greens.  At least this way he probably got a chance to taste them!

Bunny vs Food.

Simon is attempting to shove the entire hay cube into his mouth at once (apparently, they really are that tasty).  River is relegated to the cheering section and is sadly looking on with a face that says, "But I want some too!"
The bunny won in this epic battle by changing tactics and eating the hay cube like a normal bunny would instead of attempting to snarf it up in one go.
Epilogue:  Since we've discovered that Simon isn't good at sharing, we gave River her own hay cube and all was right with the world again.