Yeah, that's common on those 20 pin USB 3.0 headers from the motherboard. I know my Antec 302's cable was too thick (they've since done a revision of it) but the weight caused it to fall out of the port if the computer was moved.
Easy solution is to put a small amount of hot glue on the edge of the connector on the motherboard and slam that puppy on. With hot glue it's not permanent, at least not low-temp stuff (aka cheap) so you can pull it off with ease down the line. Just don't get any on any of the pins or block any of the holes on the cable itself (I mean a THIN line of glue. Next to nothing.)
Otherwise you can get a USB 3.0 to 2.0 adapter (I believe Rhino makes one) for about $4 and use use that.
Good luck!