In your opinion what are the reasons that let someone start dumping his bitcoins and walk away ?
I personally (and this is just my opinion) that it's more panic then simply his need of money or something else , as we saw on the BitcoinXT last time , people started to panic and simply sold their bitcoins and that caused a drop in the price .
There are people who manipulate the price of Bitcoin, and whenever these people wants to invest they do have a target price to hit. So when in pump the price of Bitcoin with a huge amount of money, others will follow immediately to make profit from the pumped price. When these people, who we regard as the manipulators or the whales, reach their target profit they will withdraw their money and that will make the price to start going down. When the price is going down, other investors who dont want to lose their money and be left with an option of having patience for price to recover will also dump when its sill early.