Why does the X-51 do a roll and then roll back at rocket separation?

I would agree with andrewcooke, but it might also be a bit back heavy. If the rocket doesn't roll over, maybe it detaches in a more dangerous way (falling forward and down into the rocket as opposed to back and away). Who knows.... I don't think there will be documentation online that answers a discrete question like this. Good question though!

my guess is that when it's rocket powered it's shielding the (blocked?) intakes of the scram jet. when it ejects the rocket booster and switches to the scramjet it changes attitude (and the "waverider" details at wikipedia are quite interesting - http://en.wikipedia.org/wiki/Waverider)