Look at the unknown line, looks like the tool used to create this graph mistook one of the windows versions for an "unknown". Likely the result of a cumulative update.
Based on the graph i expect they process data monthly, fixed the issue for june but have not gone back and fixed the data points for apr and may.
My guess would be that HTTP-GET requests were collected and the information about the OS was extracted.
If this would be true, the reason of this spike could be that one big browser randomly decided to not send OS information in GET requests and then the "bug?" was found and fixed.
This is completely speculative and likely very wrong, but:
Computer order for school districts were backed up because no one anticipated the loss due to COVID and the budget is only so big to order so many. This means we didn’t get all the replacements we needed and then suffered more losses during the school year, plus with the implementation of online school as an option. As we can’t afford name brand, like Windows (my school bought chromebooks), I assume that April and May is when the school districts put in orders for next year’s laptops.