To my mind there are two main reasons - some insider's manipulation which reasons we can never know and the global markets recession which makes people to cash out and wait for some better times in the economy.
The manipulators are not coming from the insiders but in fact it has been coming from the mtgox itself. mtgox trustee becomes the real reaons who is manipulating the bitcoin market at this time.
There are many top institutional investors and family funds who decided to cash out from the market to get the profits after the growth to the moon after the last year. I believe this trend will not last for a long time.