Well it was a synchronized pr stunt that was posted by both Rashford and Sancho on their respective accounts at the same time. With England shirt and Nike sponsoring the statement.
You could claim that this was a random PR opportunity initiated by Nike that just happened to pick them because they are both young black athletes on the team and especially Rashford has been in the media a lot for positive things lately.
A muppet would say this stunt was initiated by United together with Sancho as a way to fire back at Dortmund's media campaign. Essentially Sancho tired of Dortmund dragging it out and asking to leave without officially putting in a transfer request.