Maybe something to do with work? Afaik by February, most companies are back in action, and so maybe people are forced to switch back to Windows to do their jobs, including in some cases in their gaming rigs due to lack of work-exclusive machines.

Also the possibilities others already mentioned here sound as valid, and I second that months may have big fluctuations, but that may not relate to the growth tendency. And iirc GamingOnLinux has a graph on the growth tendency based on Valve’s hardware surveys, though I don’t have the link with me.