Rashford is today in Spain to complete a loan move to Barcelona, having been transfer-listed and stripped of his No10 shirt at the end of last season.
Rashford spent the second-half of last season on-loan with Aston Villa after falling out with manager Ruben Amorim.
Ferdinand said on his YouTube channel: "There must have been so much going on behind the scenes that we're just not aware of.
"And Marcus isn't someone who really wants to talk and do interviews, so you have to respect that.
"But I'm sure it'll come out in the wash at some point. Is it because of a lack of form? A lack of confidence? Mismanagement? Have the club failed him?
"Or is it down to him? Is there injuries we don't know about? Has he lost that desire? That form is gone and he can't rekindle it and can't get it back?
"What are the issues that have led to a young kid that's a graduate of the academy at Manchester United, which is famous for all of the great players that have come through there, that he says he wants to leave?"