Bungie hires a team of genetically enhanced spider monkeys, each hyped up on Mt. Dew and wearing a funny hat with a different playalist written on it, to scissor-kick each other until there is only one monkey standing. They then sacrifice the monkey to Sage, in hopes of appeasing him, and choose that playlist as the Double Exp weekend.
Well, that's how I think it's chosen, anyway. Here's a Link to the playlist page, where the Double Exp weekend playlist is posted every week, pretty sure that's as far into the future Bungie predicts.