I've had that issue. The dealer said one tire wore differently than the rest. He rotated the tires so that they were as they were...rotation wise originally. All was well.
On that particular vehicle I didn't bother rotating the tires anymore. It was an older vehicle and all the tires had similar depth tread.
I sometimes wonder about the necessity of rotating tires. I drive relatively conservatively and the miles I rack up per years is a bit more than average, tire depth always seems consistent on all tires...so in my particular case, unless tire rotation is necessary for tire warranty, I don't bother.
So far, no penalties in early replacement of tires, etc.