Closer inspection shows the flight departed HHN (Frankfurt Hahn).

My guess would be a military charter since Frankfurt-Hahn is close (or closer than FRA) to Ramstein and Spangdahlem. In fact on Hahn's wiki page it says " It is also frequently used for military charters, these flights being operated by, amongst others, Delta Air Lines and United Airlines."

Charter could have been for one of many reasons: one guess is a National Guard or Reserve unit in Indiana went to one of the bases in Germany for an exercise or annual tour and chartered a bird to fly the unit over together, which can be cheaper than buying individual tickets for each person.

EDIT: even closer inspection shows the flightaware link I posted is showing ATL-HHN. The HHN-IND-ATL leg isn't even showing on flightaware. Wouldn't be surprised if the leg was blocked from tracking on flightaware for this reason (being a military charter).