Actually quite a smart move by Rooney (or more likely his advisors).
He's received much credit from the fans, the club, the city, and the general public...(despite 2 quite mediocre seasons)
But, as of now, the club has only 5 contracted players (which includes 0 gks and 0 defenders)...the takeover is still weeks away (at best)...the club cannot sign anyone until that point...and when the club can sign players, they will be feeding off scraps...they may even still start the season with a points penalty (-15 ?)
Imho, the season ahead looks like it's going to be a massive uphill battle for Derby, and one where Rooney's reputation can only suffer.
He clearly doesn't need the money, and so has decided to bank all the goodwill and walk away, reputation pretty much intact, and find a decent, non basket-case club to manage.