What Could Cause This Wear Pattern?

Honestly, looks like you've gotten pretty good service out of those tires, I've usually replaced mine before they get that worn.

 

They look pretty good too me?

 

It's called normal wear and tear. Beside it seems they had there life. I would be more concern on replacing them.
When I start getting a funny wear pattern, I rotate mine, problem solved. When you buy your next set, stay away from anything that says made in China, Korea. I have stuck with Kelly's (USA) and had extremely good luck with them, but I'm local.

 

If you run long distances, remember to check pressure on hot tires, not cold. They look slightly rounded in the pics, could be a bit over. Rotating really helps the life for outside wear. When's the last time you did a 3 axle alignment? Wavy edges have usually been alignment or pressure for us. Once it was a front shock.

I understand running them thin, we do the same for fuel mileage in good weather, but I'd put new rubber on before the snow.

 

I don't see much of a problem either, but I'm sure the pics don't describe it well. As tires wear down, they go out of balance slightly, a skid, hit something, cornering, tire pressure all contribute to uneven wear. It's a universal gripe, pal, nobody likes to buy tires.