It could be a phishing scam - or it could be real. Double check your account just to be sure you aren't missing any points or certificates when this happens.
Once a little more than a year ago, I received a paper itinerary in the mail for a gentleman named Mark from Iowa flying to Edinburgh, who apparently used one of my systemwide upgrades on his M class MSP-AMS segment.
I didn't have time to track down Delta to complain - besides, goodness knows most of my SWUs expire unused anyway - but it could be that something similar happened to you.
This happened a few days after I'd called to inquire about an upgrade of my own, so I assume the agent simply forgot to close out my account before working on the next guy's ticket, and for some reason the system let her use one of my SWUs on the next caller's flight... pretty scary this is possible; I'm not exactly keen on my itinerary accidentally being mailed to another passenger.