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 .
I've seen people dump a significant portion of their bitcoin holdings because the volatility becomes too much for them. They decide they have made enough profit and want to just sit on a smaller amount of bitcoin. Instead of worrying about the day to day movements, they just want to hold what they are willing to lose in case bitcoin goes to the moon but sell everything else. I've heard people do this when bitcoin gets to certain relative highs.