I agree that it sounds more like a rack issue than a bolt issue. Assuming that the bolts are tight-tight at the eyelets. Double-check all mounting bolts all-around. The Axiom looks pretty well-braced, otherwise I'd say it could be a lack of triangulation on the rack that allows it to flex under load.

If you want the most secure bolt attachment, go with threaded eyelets backed up by a nylock nut on the bolt for redundancy, or at least some Loctite blue on the bolt itself into the threads. But I really doubt that's your issue here. Threaded eyelets are more for convenience and looks (IMO), negating the need to back up with a nut. Plus, there might not be enough room for a nut on the drive-side.