You see the matter isn't just about Sancho. It's about the perception of it. Sancho played at Dortmund just like he has at United. One good performance after 3-4 mediocre ones.
Bundesliga: Played: 14, Goals: 2, Assists: 3
UCL: Played: 7, Goals: 1, Assist: 0
21 Games, 3 Goals, 3 Assists. Hardly game-changing.
But every time he scored or assisted, the media glare fell on United and Ten Hag as if it was a crime to send an unprofessional and underperforming player out on loan. Talk shows were dedicated to The Jadon Sancho Saga. Pundits gave all sorts of opinions like this was the first time a manager and player fell out with each other. Just last night Mourinho was asked to give his opinion on the topic. Commentators made it a point to mention Man Utd in every Dortmund game. Hundreds of articles were written on the topic.
This was amplified by the Manager Out cult who will use just about any ammunition to have a go against Ten Hag. (Funnily they would have been up in arms had Sancho done the same with their favourite manager).
Even a simple thing like a step-over to win a corner was made to look like the move of the century.
And then there was Dortmund's behaviour in the last 6 months. Acting like they saved Sancho from United, insinuating United were a toxic environment for him, taking a dig at United every chance they got, all the while maintaining they don't have the money to buy him permanently. It was weird and distasteful.
One thing you need to understand, nothing unites a fanbase more than external hate. And Dortmund just did that.
I get that some United fans didn’t want Sancho to win the UCL - but the reaction from a majority of fans to their loss just seems strange to me