Seems to me it had to do with his position, as he was exactly on the spot where the ramp meets the floor; Halo has a lot of weird behavior with slopes, as seen in Rockslider's own BCM56 (clip 1, "Supergrunt", where one of the dodging Grunts slides up a slope and is launched by its own momentum) or even, I suspect, in BCM60 (clip 1, "Super-smack", where the Elite is launched off the bridge by a strangely powerful punch - I believe this may have had to do with the ever-so-slight indent present in the center of the bridge). Then of course there's the Twitchy Grunts of BCM76 (see clips 16 and 18) - that glitch is somewhat similar to yours, but not quite the same. All in all, it's really a mystery what happened to your Grunt, but we can assume it had to do with the slope meeting the floor.

Vale,
Perseus