Any idea what causes this?

The greasy hub bearings failed because of lack of lube, your rusty colored on was a combination of lack of lube and contamination (probably water). So reading through this thread, I picked up that you found contamination and replaced the fluid and seals. Was that you or did you hire that done?

Either way, one of few things happened to have a failure this soon. Either the contamination had already ruined the bearings and they should have been replaced at the time you replaced the seals. They faulty bearings overheated the hub, causing the seal to fail, creating your lack of lube situation.
Or your bearings where not set with the proper preload. This caused your seal to leak allowing the lack of lube situation.
Or your mechanic didn’t fill the hubs when he changed the wheelseal.

Anyway, there’s a good chance both cases should have been found during a pretrip inspection of the wheel seals by the the driver before the truck was on the road.

 

Hopefully you remember what that hub oil smells like. If so you’ll be able to detect a leak from several feet away. I had a left front drive start weeping and I could smell it as soon as I stepped out of the truck.

 

Leaking wheel seal usually means loose wheel bearings.

What causes loose wheel bearings? Wear.

What causes accelerated wear? Dirty oil.



No need to bring in Perry Mason, we got this covered!

 

OP, I would check your diff housing breather. That's the most likely culprit when it comes to ingressed dirt and water.

Edit: Also inspect top of housing, anywhere road salt and dirt can collect. I've seen them rot completely thru before.

 

I had one cracked on the seam of the two halves and found it by filling it with oil and parking on a steep slope for about an hour. Parked it leaning to the left first and as luck would have it, it was the left side.

 

If they are Eaton rears check the spring saddle mount welds. They are known to crack around that area as well. Seen a few friends air ride Petes and Freightliners have that happen. Lets water get into the axle ends before it gets into the pumkin.

 

Some of the ones i’ve seen are actually having the oil weeping out from the crack. If the housing is kept relatively crud free it will always be wet and oily/greasy in that location. Kinda like it gets around the breather.
One friend has 3 trucks and after he found first one leaking he used a welders dye and torch to check the other two. Only found one other had a crack in same place but no sign of leaking yet.