Diesel in the DEF tank.

So I’m a railway foreman and my crew operates a 2017 Freightliner 114sd. We were cruising down the highway when we had a derate. Truck was taken into the dealership, and it was determined that we had diesel in the def tank. I’m convinced that we didn’t pour it in, and was wondering if anyone has had the same kind of issue. Could this have leaked in from somewhere? Or did we get sabotaged? Very confusing.