It is a long shot but it may be related to the massive delays still happening at TXL, I think quite a bit of passengers from Berlin take this connection. I actually took it 2.5 weeks ago and we were delayed in DUS for about 1.5 hours.
Funny enough, we were actually only a few minutes late landing in DUS but there was some mechanical issue with the plane.
IIRC, there is only one non-stop flight from DUS to BOS daily with AB (AB7472 @ 15:15 - 17:10). The returning flight BOS > DUS (AB7473 @ 18:55 07:45)
Looking at FlightAware (or similar site) almost every time the flight from TXL > DUS was late, so was the DUS > BOS flight, which would make the return BOS > DUS late.