Re-emerging at EMA is conjecture- they have a number of flights to Leipzig from there already so unlikely to be adding another one as a direct replacement. DHL have taken at least 3 757s out of service in the last few months with only 1 replacement coming in. There are supposed to be 2 more under conversion which would presumably make up the gap, but they haven't been delivered yet.
A more likely scenario is that they managed to work their schedule to operate flights for the busy couple of months up to Christmas, with the help on occasions of the Bulgarian 737s, but now that things are quieter for a while they have moved the capacity. It's always possible that having dipped a toe in the water they've had a rethink, but apart from spotter conspiracy theories I struggle to come up with something the airport may have done that would scare them off, especially with them being such a big MAG group customer.