Last week I was fishing using braided line with a 3' Flourocarbon leader.  I felt a tug and my line immediately went slack.  I reeled in and found that my leader know was still there and about 1/16" of leader.  There are known to be Northerns in this lake.  Is it likely that a Northern actually bit off 3' of leader line?  Or is there some other possible explanation?

Probably had a nick in your line. The tug was enough to break it at the damaged area.

Pike sometimes bite lightly, literally slurping resting jigs and plastics, and often its the facial scales that wear out the line, not the teeth.  It you were using a contact bait in rocks, perhaps there was some abrasion, as mentioned above.

Where I fish there are a lot of muskies. I've had the same thing happen a few times, feel a hit and reel back nothing but line.

I don't think there are any muskies in this lake, but as I said, there are northerns.  I was just surprised that so much of the leader, a full three feet, disapeared all at once.  It could have been a nick in the line, but I was fishing old pad stems, not rocks.  Just wierd.