It's an objective reason here, as you can see, Poloniex added today a warning about lisk and the price started to decrease immediately. I didn't expose my emotions in this post, though I should've been.
The same happened this night with STEEM as soon as they changed the warning about a date when it will be available (as I explained here). It doesn't matter if they do it intentionally. What matters though is that they address the technical issues in the worst way possible. When a service is bad, the customers shouldn't be silent about it and the issues should be solved!
RE: How Poloniex manipulates the price