Well.. Bobby's a boy and Weasley's a girl, so maybe the attacker didn't want to curse a girl, so he just knocked her down instead?

Or maybe it is because of what Bobby said - not that I'm saying Marvolo had anything to do with it or anything, but maybe it is someone who thinks they're doing him a favour?

Or Li and Brocklehurst are right, and someone's out to get the halfbloods.

I don't know. It could be anything. But I know Bobby didn't deserve it.