NRGY is a victim of tax selling. It is one of the few MLP's that has performed poorly so one of the few you can sell for tax losses. Look for it to rebound after the first of the year. The insider buying is more significant than any of the other 30 MLP's I follow and the company has a good plan for getting things going in the right direction and is implementing. I am a buyer here.
If you own a bunch of MLP's, most trading at or near their 52 week high, and you want or need to sell, which are you going to sell? Depending on how long you have owned Inergy and what price you bought it at you may owe no tax at all or more likely get a tax loss to offset gains. Picking the flowers and watering the weeds is always a bad strategy but as you point out, especially bad with MLP's given the tax consequences. Thus Inergy becomes a likely sell as compared to other MLP's.
I personally think the main reason for the big move down is the standard over reaction to the lower than expected results and the fear that things are worse than the company is saying. The insider buying mitigates that for me. Also, the one time charge made it look a lot worse. However, I do think there is tax selling as well. I just bought quite a bit yesterday so that is my working theory! There was a time not so long ago when Inergy was very highly regarded. My bet is they will navigate this bump in the road without any long term damage.