You can also tighten up the receiver with two bolts drilled through the male and female pieces, one vertical and as far from that hole as you can with one horizontal.
It's because of the slop in the receiver tube.
Yeah another good option for bike racks that you might want to lock is this: Let's Go Aero (SHP2040) Keyless Press-on Locking Silent Hitch Pin for 2in Hitches Hitch Lock
It's very similar to the Curt but locking. I use that in combination with the U-bolt shown at the beginning of the thread and the rack is extremely firm. I honestly think any remaining wobble comes from the 100 series hitch which isn't super stout.