Mandelson just quitting his cushy job with the EU to cosy up to his new best friend Brown didn't make sense. Now the reason is becoming clear. The net was closing in on Lordy Mandy and Brown couldn't afford such a dangerous loose cannon on the loose.
Just two weeks ago, when Brown surprised everyone by putting out the welcome mat for his arch-enemy, the Orange Party reckoned Brown did it to keep his friend close but his old enemy even closer.
But even that told only half the story. Brown had tried to prise Mandy out of Brussels when he first took over as PM, only to be told he didn't have the authority.
Journalists placed bets on who would be first to bring down the master of spin. They didn't need to. Old habits die hard. He was doing it all by himself. Where Mandy goes sleaze is not far behind and often it's the other way around.
Mandy had been in their sights for months, with his links to a dodgy billionaire Russian mafia character, Oleg Deripaska.
Deripaska is banned from the US, sued in London by a fellow Russian and pursued around the globe by investors.
Getting close to the fabulously rich and enjoying all the fun of the £80m yacht off Corfu is all part of the Brussels way of life.
But while, with one hand being asked to investigate this Russian guy and with the other, smoothing the way, as EU trade commissioner, to cut aluminium import duties to secure huge aluminium contracts for his pal, that was too much even for Brussels to stomach.
The Mail reports: "Deripaska benefited to the tune of up to £50m a year from decisions to cut European import duties on aluminium, taken in 2007, also while Mr Mandelson was at the helm in Brussels."
Just booting out Mandy would result in one almighty sleaze row and Brussels likes to keep that sort of thing under the carpet. And he would have lost all the pensions, perks and pay-offs. With a leadership challenge from the Blairites just on hold, Brown couldn't risk this bitter Blair buddy trying to stab him in the back.
So strike the deal. Mandy leaves with his head up, sort of, and keeps the fat pension and pay-offs. Brown takes in public enemy No 1, on a promise he'll keep his head down and not cause trouble.
One of the first things Brown did when Peter took the peerage was to strip him of the anti-corruption portfolio that went with the business secretary cabinet job. If he trusted him so much and there was no whiff of sleaze following him from Brussels, why this odd move?
Mandelson and Brown may think their new love-in will have a happy ending. It well prove to have a happy ending for Brown. Without the safety net of Brussels, he can now just boot him out. That would mean third time unlucky, total disgrace and total political oblivion and no longer a threat.
No one - not the Mail, Telegraph or Times is finished with Mandy yet and no amount of spinning will get him off the hook.