Wobbling Topeak QuickTrack rack bag
As an interim answer, it sounds like the plastic track has been wallered out and now has play between it and the metal track.

  Topeak UNI Super Tourist DX Disc Carrier stock photo showing rail in the middle

Topeak MTX Trunk Dry Bag stock photo showing underside.

First guess is that the grey plastic foot has worn through insertion/removal over time.  I doubt the metal has worn, but it may have unfolded a little.

You need to look at your bike and see which is the cause.

Your options are:


Check that the foot is not wobbling on the bag - as ChrisH points out they're only rivetted together, so it may be sloppy there.  Drill out the old rivets and install some small bolts with washers, and nyloc nuts to stop them coming apart.
Pack the gap between plastic and metal using a strip of additional plastic.  I hear sugru is the bee's knees for this kind of repair but it probably wears even faster than the original.  Or you could use a long thin piece of brass or aluminium, which would wear faster than the steel, but may need one on each side.  This might be called a "Gib" in the machining world.
If the metal runners are bent outward, you may be able to bent them back in with pliers or a vise or a big clamp.  Risk is snapping the steel, or potentially weakening it.  I doubt this part is available as a service item.
Replace the plastic foot completely.  I do not see this as a service item to purchase, which is somewhat surprising.   You might try to 3D Print a replacement, after taking careful measurements of your existing one and of the rack track.  Or if you lean more to woodworking, a hardwood rail with the right dimensions could bolt on in place of the worn plastic one.  You could mill the new foot out of aluminium too, depending on what access you have to tools.