Why is this a war on Christmas?

The other day, I was sitting with a bunch of my friends and one of them wished us happy holidays as she was leaving and another friend flew off the handle and said it was “a war on Christmas.” He then proceeded to say that we (everyone in the room) was Christian because we all have been baptized so we all should be celebrating Christmas (I have never been baptized). Why is wishing happy holidays instead of merry Christmas a war on Christmas?