Lots of them are doing it here, too.

I think it's because it got too cold too fast. Although they had turn color, they were still alive when we had several nights in the teens in early to mid-November. When that happens I think the plants often immediately just go dormant, and not even produce the abcission layer that they need to shed their leaves.

The leaves will either fall off gradually through the rest of the winter, or, more likely they will simply shed around the time of bud swell in spring.

Mine have always done it a little bit, yes it might have been worse or more notcieable this year. Usually the first couple snowfalls helps get them off, but the snow we had this year was very light and powdery when it was falling.
It also varies depending on the cultivar.