People loosing confidence in it. With no one & nothing backing it there is nothing to prevent the value from going to 0. The only thing keeping the value of bitcoin from being 0 is the willingness of people to trade them for other things of value.
Ultimately, that's the only thing keeping any currency from going to zero.
Well, on one level, yes, but in practice there are many organizations and governments that would step in if a national currency were headed towards a value of zero. Bitcoin doesn't have the same level of institutional support (yet?), and so if it's value starts crashing hard, it could turn into an unrecoverable death spiral.