Wow, this is interesting. Tae also does this, usually when she's happy or excited. When I arrive home from work, we go upstairs so I can change and she rolls around on my bed nibbling on the blanket. She also does it to me when we play.
I have always attributed this to a lack of bite inhibition. I got her through rescue, but was told by her original owner that they took her home at 6 weeks. Since bite inhibition is learnd between 6 and 10 weeks, I assumed it was that.
The bad thing is I tried to take Tae to day camp at Petsmart and they took her out of the room, saying she was nipping at the other doggies. They said she seemed excited when she did that, so I figured it was the same kind of thing. She never did so at the doggie park when we went, only at day camp.
Oh, I forgot to add that the behaviorist did tell me this is lack of bite inhibition, but maybe it is a Boxer thing? She told me to stop the nipping/nibbling by saying "ouch" loudly, then walking away and ignoring Tae for 5 min. Only problem is she normally gets nibbly with me when I get in bed at night and at that hour I hardly feel like getting up and ignoring her for 5 minutes!
Since this sounds like a Boxer trait, I'm not going to worry about it anymore!