bike wrench

Yeah it doesn't seem intuitive, but the bearing inner race is likely the hardened axle and follows the curve. It is a very common problem on cone bearing hubs, and it is the most likely culprit. It is impossible to say for sure without being present and taking it apart to see. I could be wrong. I have not worked as a bike mechanic where I have a ton of direct experience with these specifically. I can fix anything when present or on the road, and have extensive experience with cars, hot rodding, CAT/Case/John Deere heavy equipment, metal fab and machining, not to mention PCB design/fab and FreeCAD/3d printing... Not trying to brag, I'm just adding context. I make mistakes all the time like any human. I have only ever had to replace one bent axle in a rear wheel in my life, but I've seen issues mechanics had in passing, and been tasked with tracking down a replacement for them dozens of times. To my best recollection, the only reason for this kind of wobble was a bent axle. Always be skeptical though. "Mod" means nothing to me. I'm just the janitor.

It can happen if the bearing wears out. It has to be occasionally cleaned, regreased, and sometimes needs new rubber seals, or just replacement. Friction-related heat can definitely warp an axel housing. If it’s bad enough the wheel will seize. I wouldn’t be too surprised if there was also lateral wobble in the wheel.

A cross-threaded cassette might also produce this effect.