Here's me taking off my mod's hat and hypothesizing out of whole cloth.
5E annoucement was planned and ready to go as an Apple-style "And one more thing..." button to the Sat product seminar. 5E is going to be a minis-based skirmish game, sold in non-random packs, with optional rules for leveling up your minis through repeated play. It will go one sale at GenCon 2012. Someone decides at the last moment that whatever they're doing doesn't need a year's leadup, and they want to save the big annoucement for D&D Experience in Feb 2012, where they can manage the whole marketing rollout of the new edition and do a better job of manging customer reaction over six months rather than a year. (As well as keeping sales up for the handful of 4E products it has just announced.)