How does the market bubble? Before the burst due to massive DDOS attacks on most Bitcoin trade sites, the market price rose from 30 USD in January 2013 to over 250 USD in April.

The mechanism of collapse - the DDOS attacks - is pretty clear and transparent. I wonder though, what mechanisms, machinations, influences or other external sources influenced the market that caused such bubble growth in the first place.

It appears the market is still vulnerable to such (or similar) mechanisms (currently we seem to observe a similar growth), and I'd really like to learn how such events work - how do such bubbles of price appear and grow? Are they natural element of the market, self-propelling machines of enthusiasm of masses, possibly following some global events/trends, or are they created by subtle, or less subtle manipulations?
The short answer is momentum.

Imagine for a second that you are a neutral observer and you see that the price of Bitcoin has been rising over the past few days. Even if you have no reason to think it will continue to rise over the long term, it makes sense for you to buy Bitcoins. After all, it's unlikely that the rise will end just as you buy, and if the price starts to drop, you can always sell. Your buying puts upward pressure on the price, maintaining the momentum.

Now imagine the price is falling. Even if you believe Bitcoins will go up over the long term, it makes sense to sell now. You can always buy your Bitcoins back for less if the price starts to rise. Your selling puts downward pressure on the price, maintaining the momentum.

So, essentially, if the price displays momentum, it's logical to assume it will continue to do so for at least a short period of time, and thus it's logical to perform a transaction that contributes to that momentum.

Negative publicity can start downward momentum. Positive publicity can start upward momentum. Momentum can't really be maintained forever though and when upward momentum stops, everyone fears that downward momentum will begin. They sell to take advantage of the high price and start downward momentum. The same is true when downward momentum ends -- everyone who is bullish over the long term wants to take advantage of the low price. This starts upward momentum.

Just be aware, trading on momentum is essentially a zero-sum game. Everyone's trading profit is someone else's loss. You are probably not "smarter" than everyone else and so are just as likely to be a winner as a loser.

However, the long term upward trend is not a zero-sum game and can mean that there are more winners than losers. Essentially, people who see Bitcoins as an investment are hoping that the Bitcoin system creates real value over the long term.