- The Dwolla/MtGox BTC <--> $$$ link breaks, perhaps due to pressure from bank regulators in response to Paypal or VC/MC lobbyists.
- The speculators who are buying BTC run out of money/the bubble bursts, as Genrobo mentioned
- Someone hacks MtGox or some other key BitCoin participant to steal BTC balances
- Someone identifies a cryptographic fault with the Bitcoin scheme
All of these scenarios would cause a crash in BTC values. I guess you can argue they are unlikely - at least the first two seen quite plausible to me.