The source is his own and his father words. It was Haaland´s own decision to go to Dortmund, and he was never rejected by United. He saw Dortmund as the natural next step, matching his development (Molde->Salzburg->Dortmund->?) (just like Bellingham). In hindsight, it might sound strange that Haaland was worried not to get game time at United, but he was playing in Austria, remember, and Martial was on fire. Still, If Haaland had trusted that Ole would remain at United, that could have helped, but Ole´s job looked everything but safe January 2020 (there was even a story circulating that Haaland requested a guarantee from Woodward that Ole would stay in his job, but that of course, could be just bollocks). So according to Haaland himself, he went with this gut-feeling in the end, which could be a mix of many things. Would he be starting, did the club really want him (e.g. the fans, the board), what would happen if Ole got the sack, how would the next manager treat him. There is one thing many new appointed managers seem to be cautious about, and that´s players with too much affection to the former one - just look what happened to the Norwegian players Ole signed at Cardiff. These things counts.