Why do my hubs keep wearing like this?

This is the hub I just removed from our 07 Mazda3 race car.  This has 4 endurance races in it, so about 3500 miles.  It was a good used hub when installed.  The inside half has basically been machined down, causing everything to loosen up.  I've had several do this in the last year or two, and had it happen on our last race car as well (Civic).  
 

Why is this happening?  Am I doing something wrong when I install them?  I'll usually put a light smear of grease on the hub when I press them all together for ease of assembly, is that causing problems? How do I avoid this?  
 

Whenever I see wear like that "in the field" it's because they drove on it for a very long time with a noisy wheel bearing, and the constant vibrations destroyed the hub/race interface due to fretting.  Steel is remarkably flexible under load, and press fits stop being press fits and things will shift around a lot.

I'd try something like moly paste between the two or maybe a high metallic content antiseize like the fancy copper stuff for oxygen sensors, or nickel stuff for Ford 3v spark plugs.  The metal content may cushion things and prevent fretting, assuming that it doesn't just get wiped off when you press it together.  A tighter press fit would also help but that's not something you can accomplish...

I'd also try increasing the torque on the axle nut.  Are you also seeing similar wear between the face of the inboard inner race and the face of the outer CV joint?

Ultimately the cause is insufficiently strong wheel bearings allowing everything to flex under the strain of racing.  You may just need to life out the hubs on a regular basis.  I recall a story of when Ford engineers took a preproduction SVT Contour to a 24 hour race and they had this kind of failure at basically every driver change... the production cars got larger bearings, apparently.

I think Pete nailed it. Short service life for racing applications. Another "fix" might be to retorque between events to take up any wear/deformation that has occurred. That might extend service life but probably not more 2x.

I see that Timken and Mevotech make some heavy-duty hub assemblies for the '07 Mazda3.

Interesting.  Certainly our use case is out of the OEM design specs. This one only had a bit of play, and had no play before the Sunday session of 7 hours. 
 

I'm torquing to 200+ ft/lb, the max of the ORM spec. The axle nuts are marked, and the one in the first picture hadn't moved.  Sometimes I get the axle nut backing off, and when I do it is always the hub wearing.  
 

Some of these last a year+ (4-5 races).  Some last one race.  The inconsistency is what bothers me. Both for OEM and standard aftermarket replacement. 
 

In replacing this one today, I tried no grease putting it together, but put the hub in the freezer for a while before assembly.  Let's see how it goes.  

I think what is happening is that under cornering load, the end of the hub is precessing in the bearing because the bearing isn't beefy enough, and the part of the CV axle that holds it together is stretching under cornering loads.  The whole assembly flexing is allowing the hub to wallow around inside the bearing.

Lack of grease may actually make it worse, since the wear is caused by metal on metal friction.

Do you check the torque on the axle nut or just mark it?  I'd check the  torque AND mark it, so you can see if the axle is stretching. 

Now that I remember more, in that SVT escapade, the axle nuts were getting "loose" even after they were welding the nut to the axle.  Then they broke the end off of one.