There is no crew base in BOS any longer, so here is my speculation... a JFK based crew flies up to BOS from LGA on the Shuttle to meet the flight and relieve the original crew. The plane pulls up short in BGR instead of BOS, and DL dispatch orders the relief crew to BGR via a DL shuttle 737. No DL connection flights are scheduled BOS-BGR between 9AM and 3:18 PM.

Purely speculation.