I've been reading a lot of new studies on car usage lately, and I think the biggest thing is that there is no one big thing. It's a bunch of things that add up to a major shift. Reasons I've seen, and all seem to make sense, but I'm sure most people would argue at least one of them.

The economy (although the downtick in driving started before the economy tanked)
Gas prices
Technology rather than car focused millennials (licensure rates are crazy low)
Retiring baby boomers
Technology is enabling mass transit. You can see where your bus is, look up schedules of multiple systems and nodes with singular apps/tools while on the go.
Technology is also enabling car sharing.
Telecommuting
The ability to order anything you could possibly imagine and have it delivered to your door, while price checking that item against the entire country
Average driving speed in America has been slowing
Genuine concern for the environment
The number of protected bike lanes doubled in 2012 and are supposed to have doubled again in 2013.

Urbanization (or gentrification) caused by:
20 somethings waiting to start families
Since the housing bubble boom, lots of people don't feel comfortable buying homes. They've heard all the horror stories of being upside down on a huge loan, so they're renting.

One of the most interesting theories I saw about why it was impossible to match the rapid gains in vehicle miles seen over the last 50 years is that women entering the job market has been saturated. Since the 60's more and more women have been entering the work force until we reached a point where if a woman wants to work, culturally there's no problem with that. This wouldn't decrease miles driven, but it would slow down the rapid increase in miles driven that we've seen historically.

I don't think the downfall of cars will be cars, I think it will be the roads themselves and paying for them. We've been building so many roads so quickly for so long that the repair bill makes building more roads arguably cost prohibitive. And if more roads aren't built, then there will be a further decrease in travel speed and another decrease in automobile usage. A minor shift or hiccough could seize the vmt->roads->vmt engine. But if we aren't willing to pay the repair bill of 1.5 trillion miles of roads + constant road expansion, we certainly won't want to see the bill for 1.7 or 2 trillion miles of roads + constant road expansion, especially if vmt is unstable.