The article below provides a great explanation of why this is happening as well as gives instructions for how to stop it from happening to you (or rolling it back if it already has).
Why Microsoft forced Windows 10 upgrades last Friday (InfoWorld, 01/23/17)
NOTE: You may want to accept the update because Microsoft will be ending support for the original release at the end of March, which means there will be no more security patches or other fixes for that version after that point. However, you probably still want to schedule the update rather than have it happen when you’re in the middle of using your computer.
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