I don't follow it closely like you but I'd look at the chart relative to the US IRS ruling. Prior to that BitCoin had a lot of potential as a currency and financial asset. That ruling really knee capped the currency potential and thus limited the upside.
I don't really think the IRS ruling had anything to do with it. Whether it's legally property or currency, people will be able to continue using bitcoin as they always have. The only thing that changes is what they report on their tax forms at the end of the year.
I really don't think speculators were thinking that far ahead -- herd mentality is a way more believable explanation, there are hundreds of examples going back to http://en.wikipedia.org/wiki/Tulip_mania
As a group, speculators still need to have some underlying theory for why the vehicle they're speculating on is valuable. Almost by definition, a bubble popping is when speculators collectively recognize that theory didn't pan out.
For BTC, probably the most likely theory is, "Bitcoin will become valuable as a currency." The world's biggest economies officially deciding that it is not a currency under their laws dealt a crippling blow to that theory.
IMO, what makes it a 'bubble' is that the speculators don't care about the underlying theory, they're just jumping on the bandwagon because it keeps going up. Which is why it crashes so suddenly as soon as forward momentum slows down.
