Blass, Debrunner, and Funk refer to this verse in section 250 of their grammar. They say hOI DE means "but others" and states that unlike the other examples it list that start with MEN and then are followed by DE that:
BDF wrote:in these two places (the other being Matthew 26:67) no differentiation is indicated at the beginning of the sequence, but with the appearance of hOI DE it becomes evident that what was siad first did not apply to all"
I know that quite often in Classical DE is simply used to denote a new sentence or idea. It could be that Matthew is Atticizing here. Just a guess.
Small sidenote....I was just saying how great I thought BDF was....and here they are the only Koine Grammar that deals with this specific verse. It could be that since they are writing a Koine Grammar coming from a Classical standpoint, that this sort of thing stuck out a bit more to them than some of the other modern grammars. Bert...I'm telling ya....if you don't own BDF you should!