BP in Short-Selling Free-Fall

A month ago, Wall Street rushed in to support BP shares. As a result, while shares had lost a good deal, the falling knife had not commenced. With the past two days' plunge in price, shares are now in free-fall.

But you shouldn't think institutions are selling as much as they are shorting.

In my opinion, most of the institutions unloaded the shares they had planned to sell due to the event a few weeks ago. Now it's the shorts which are causing the stock to collapse.

Today, I picked up a (very) small percentage of my total intended position towards the close of the trading session.  I did so with the thought that shares will head lower. 

Why did I buy then?