Tom Watson said he had been thinking about quitting as Labour election co-ordinator "for a few months", because he found the post very tiring and felt unable to speak out on issues of concern.
He told BBC Radio WM: "We have had all this fuss over Falkirk and I just thought to myself, if I am getting into the story and not being useful any more for Ed, then it's time to move.
Asked about the details of what had happened in Falkirk, he said: "I really don't know about that. One of the things I did on Falkirk was keep out of that process, because there's an employee of mine who was a candidate.
"I know this works - people put two and two together and very often get five, but that's not the reason I have gone. The Falkirk thing crystallised in my mind how difficult it would be for me to stay in the post."
He added: "I've had an unusual journey in politics. There are some people who have never quite forgiven me for resigning in 2006 under Tony Blair and I accept that completely. What I don't want to be is a problem for Ed Miliband, so going to the backbenches is the best thing.
"I want him as Prime Minister and he has got my full support."
Labour leader Ed Miliband has turned on the trade union Unite - his biggest financial backer - whose votes helped him become leader.
Tom Watson has announced his resignation from the shadow cabinet, amid a row about the influence of the union Unite on the Labour party.