As other’s have said there must be more to this than meets the eye....
While they were still sitting on the plane at the gate at IAD, the flight deck crew told passengers the delay was due to their request for a software update for one of the engines, which the crew described as running faster and unable to sync with the other engines.
TBH I certainly can’t imagine and have never heard of flight crew requesting a “optional” software update downroute. As has been previously mentioned one option might be the aircraft had an engine issue inbound to IAD which resulted in somebody on the maintenance side of the operation ordering a fix (possibly software related) prior to the IAD-LHR sector.