“For some reason” could be “just because,” since he does have a history of changing up his hair pretty drastically. He’ll color it, cut it, shave it, basically anything that can be done to hair.
This latest style is a totally shaven head, but his full beard remains. His mother posted a selfie of herself, Zayn, and Gigi Hadid, presumably right after the hair disappeared.
Of course this was just a couple days after she posted a picture of the group when Zayn had a buzzed-style cut. Same day, but really a latergram?
And for review, here are some of his styles over just the past year or so:
It must grow pretty fast.