The reason it was moved was because at the time Saints didn’t have a 15k capacity which was the threshold for holding big games e.g. semi finals in Europe.
As i recall, a couple of Prem games were moved as well. Nowt to do with europe. And bumping into some egg chasing mates down The Rover, or The Foundrymans, they just said, its a day out, and we are having a beer in Bletchley rugby club. They were moved for money. Nothing else.
https://www.northamptonsaints.co.uk/news/saints-versus-saracens-heading-back-to-stadiummkhttps://www.northamptonsaints.co.uk/news/your-day-at-stadium-mk