The downgrade was by Credit Suisse and was very specific in nature - the cancellation of an acquisition which was going to help them get back to 1.0 distribution coverage. So now the possibility of a distribution cut has increased.
My take is that the company had buried the news of the termination of the agreement in their 10-K and until Credit Suisse highlighted it, hardly anyone was even aware of it. So the plunge was as much due to the news as due to the downgrade. Since it was in the 10-K buried on page 46 or somewhere nearabouts, the company had clearly known about it for a few weeks but not thought it necessary to make any announcement. Pretty shoody treatment of unitholders by management, if you ask me. That would also be a good reason to sell.