I believe that through exasperation with what's going on in the party be decided to stand. He described to the local clp that he wasn't part of the coup, and I believed him. I think that he realised that if there was to be a contest and a very public debate about the shape of the party, that being part of it he could (can) try to shape the debate around what matters for the membership eg making the party a genuinely democratic socialist one. I believe that he believes that in having a debate where the terms are about what the membership cares about, there's a chance that the party might hold together. If one of the blairites, or red tories, or right of the party had stood, the debate would have been completely different and the party would definitely split. If he'd acquiesced on the matter, he would be part of the future Corbyn leadership, but that would be as part of a split party. As it stands, it probably will still split, but he's trying to do something to stop it.
The silence from the blairites in regards to supporting Smith is telling as to where he stands in the party. I believe that for all the noise going on around the debate, you've got a guy who wants to save the party he cares about.... Save it from the right of the party as much as the left.
If I'm naive, or if I'm trying to control the debate, I apologise. I just tried to correct a falsehood about my local MP.