Many passengers aren't aware that buried in the conditions of contract with DL, is language similar to the effect that they "may divert you to other modes of transport".
I've had air freight shipments diverted to rail carriage for the same reason.
I used to hear that a lot of flights to LEX out of CVG ended up being bussed.
Only happened to me once, when trying to get to PSC. Bussed in from GEG due to fog