There are a few reasons why you might want a train to run early.
Maybe you are missing a member of train crew so you grab someone who is passing on an earlier train and get them to work the service instead However it might be preferable to make sure the service out of Euston runs on time so they run it fast into Euston to protect the peak out of Euston
Or a member of train crew has agreed some overtime but must be on a certain train back to book off. Better to run the train fast for some then to cancel it for all.
Or there is an issue with the slow lines so it might be better for the overall service if it gets diverted onto the fast lines but runs fast. The signaller might be more willing to put a non stopping service out on to the fast line then a stopper.
Or the unit had a safety related defect at the north end, got swapped at Rugby to go back south which is OK. Then you either terminate it at Northampton for fitters but that has displaced train crew for the peak and would be a short form and seeing how this timetable is probably loads of delays and cancellations too.
So maybe they run the service to Euston and try and bury the defective end. Maybe they searched around for different ways to bury the end with various unit swaps, probably not helped by the fact there aren't many four cars into Euston at that time of day plus a lack of platforms to hide units in for extended dwell time and the only way was to run it fast to Euston to get it in front of the unit it is booked to couple up to. A bonus if that unit stays coupled until the end of day.
Any of those are possible including forgetting it is booked time somewhere but I guess as there was a swap at Rugby I would go for the unit fault....
