Most bubbles start by accident -- this one was carefully engineered.
How do you engineer a bubble? First you invent an asset. But if it's an asset that only you own then nobody will want to believe it's valuable. So you make it possible for the first round of early adopters to get it for free, by "mining. The first adopters talk the second adopters into trying it too (they can still get significant amounts by mining) and... well, you know the story. You also wrap it in some political anti-fiat-currency anti-bank anti-Fed anti-government anti-whatever pro-cryptography political wrapping to make it appealing to a certain constituency. But the key thing is to give out the "asset" for free at first, and to a lesser extent as time goes on, to incentivise early adopters without requiring 'em to actually put their hands in their own pockets. It's quite brilliant, the best memetically-engineered scam since Scientology.
And now I'm really getting tired of discussing bitcoin. I think I'll write a big long blog post in the form of a FAQ, submit it, and never mention bitcoin ever again.