No clue. Got the car 3 1/2 months ago and they were in there. The outside edges of the tires were pretty worn then and seem to be getting worse. The back tires have great tread all the way around. So, several possible things going on here:

- maybe the back tires were also worn like this and they only changed the back ones. Not likely.
- maybe the control arms or some other components are bad.
- may just need an alignment.
- maybe normal wear because front tires have been on there for 40k or more. Doubtful.