Each of these boomsand, for that matter, most bubbles throughout historyhas been driven by the same basic publicity-price feedback loop. As an Internet writer, I've seen this process first hand. During times when the price is rising, there's a lot of traffic to be had writing about bitcoin, so reporters like me write articles (like this one!) about it. The articles cause more people to pay attention to the currency, and some of those people decide to buy. That pushes the price up even more, triggering more media coverage and more public interest.
This is very true because it is what drives most of the hype about new things. Then when so much interest is on a specific thing, everyone and their auntie (well not quite with bitcoin--yet) wants to buy it. So then it gets more and more expensive.
That's also the issue in the other direction, when people realize that speculation is just... speculation. That the numbers aren't supported by the facts, and the prices are much too high. Then they all run for cover, sell it off, and protect themselves. The recovery stage takes time, but it will go back up again.