This explains the most recent drop in Ethereum price:
The current soft fork deployed in Ethereum poses a DoS vector. If the soft fork activation goes ahead as planned, the community should be prepared for potential DoS attacks, which would lead to diminished performance for the network.
Specifically, an attacker can flood the network with transactions that execute difficult computation, and end by performing an operation on the DAO contract. Miners running the soft fork would end up having to execute, and then subsequently discard, such contracts without collecting any fees.
Source: http://hackingdistributed.com/2016/06/28/ethereum-soft-fork-dos-vector/