I downloaded and looked at the raw data. The biggest reason seems to be the new Edge browser gaining popularity. It went from 5.8% to 7.4% market share since October. I'm not sure why this chart displays both IE and the old Edge, when together they're a third of the market share of new Edge.
Safari and Firefox also are up since October, but I'm not sure why that is. For Safari I suspect new Apple devices being purchased around the holidays, but that's just a guess.