The relationship had been troubled for a long, long time. Remember that General Electric already owned NBC when Disney went out and bought a direct competitor, ABC*. There are "rumors" that a tiff developed over the required refreshment of 'Horizons'.
There's also the whole background story of corporate sponsorships at the theme parks to begin with. Original, they were simply advertising vehicles for the companies they got their sign on a building and the association with the strong Disney brand. Companies used to clamor for the chance and even did things that were considered unthinkable remember when both Coke and Pepsi had places in the parks. Try to think of any other place on the planet that let you buy both**.
But things change. The value of being associated with Disney has declined and the prices keep going up and up and up. To keep the sponsorship money flowing, Disney began to make the sponsorships more "business deals" than "advertising". The reason we all suffer with Nescafe coffee on property is because part of the sponsorship deal for 'The Land' requires that Disney-owned properties buy Nestle brand products exclusively. It's why Exxon gas stations are spouting up all over property. Why soon every computer will be sporting an HP sticker.
The most interesting example is McDonalds. In exchange for the continuing relationship and funding the Dinoland area of Animal Kingdom, McDonalds was basically given a free hand in the parks. Full fledged McDonalds with their menu opened inside both AK and California Adventure; fry carts/stands opened everyplace else. The companies understand the advertising isn't worth it anymore. They're goal is to lock in a major corporate customer. The public aspect of things (like rides) is an afterthought.
That's why major companies that don't need Disney as a large customer are bailing. There' s not a lot of items that United Technologies can hope to sell Disney so they left. And there aren't a lot of power plants or jet engines that GE could sell either so they bailed as well. The public exposure of being a sponsor at WDW isn't worth Disney's asking price and they Disney isn't a big enough customer to warrant any special attention***.
* - okay, history has shown that ABC really isn't any competition for NBC, but most people assumed Disney wasn't going to run their network into the cellar the way they did.
** - the deal became exclusive as part of Coke's sponsorship arrangement for 'The American Advenure' when EPCOT Center opened.
*** - I should make the joke that Disney isn't much of a customer for light bulbs now either. But that would be too obvious.