I thought that the value of Bitcoin drop due to the hacking of bitcoin system? The hackers are still there on look out. But still bitcoins value changes from time to time so dropping or increasing is not really that big.
There are many reasons behind this. First reason is government decision .when a government of a country ban bitcoin in their country the international market price of Bitcoin is also fall. 
Many government ban bitcoin in their country because of Bitcoin decentralised nature and its involvement with criminal activity. Another main reason behind this price falling agenda is media. Many media talk about Bitcoin negatives and this thing affect people trust in Bitcoin. And ultimately results the price of Bitcoin is fall. When the financial services agency suspend the crypto currency exchange in a country then the price of Bitcoin is fallen. 