Sent my upper back after my groups opened up suddenly... Like 10 moa... There was no key holing. They replaced the barrel which seems to have taken care of the problem. What could have been the issue with the first barrel?

Was it chrome lined? Lining flaking could cause that. Damage to the rifling at the muzzle is another possibility.

Stainless.. Was thinking muzzle too but I would think that would cause keyholes.?  What about the barrel extension? Could that have become an issue somehow?

Backstory on the upper was that it is a PSA upper with a Midwest rail. When I bought it, it came with an improperly torqued barrel nut which became an issue. I set it aside then eventually got around to properly torqueing the nut properly. But too late, it was shooting like crap, and I'm not sure where the issue was. (And thanks PSA for fixing it under warranty) Someone suggested that the loose nut could have banged up the front of the receiver affecting barrel/receiver lock... But like I said, they replaced the barrel and it seems to be GTG...(unless they lapped the front of the receiver when they serviced it)..

I would think about serious crown damage or muzzle brake.  
Otherwise an irretrievable rifling problem, but I don't think of that happening suddenly.
Let us know what you find out.

barrel nut came loose..........will cause one to open up groups real quick........

Yeah, but I broke it down and torqued the nut to spec. It was shooting 10moa afterwards..

Not gonna find anything out now, the barrel is gone, I'm just speculating on what the issue could have been.

No rifling issue, the barrel probably only had about 500 rounds through it tops. And if it was crown damage with 10" groups, would there not have been keyholing?