Knowing nothing about this, I wonder if they're getting ready to retire Windows Server, and wanted to get their server products off it?
Edit: How they did it is also quite fascinating:
https://www.microsoft.com/en-us/sql-server/blog/2016/12/16/s...
https://www.microsoft.com/en-us/research/project/drawbridge/
>a key contribution of Drawbridge is a version of Windows that has been enlightened to run within a single Drawbridge picoprocess.
MSSQL on Linux only seems to use parts of that project (a smaller abstraction layer), but that's still super cool.