Zacks' ratings are based on technicals, and earnings beats and misses, and analyst increases and decreases in earnings estimates. Basically, a computer reviews these things, and spits out a rating. Its a bit of a 'momo' ranking, although I believe their algorithm looks for technical basing, and will upgrade if a stocks tanks and then bases for a long time.
While I'm sure they have individual 'analysts' each assigned to many many stocks to review what he computer spits out; they arent' analysts who actually study and investigate the fundamentals of a company.
So, I would gues that the 'strong sell' rating is based on some technical breakdown and/or a recent earnings miss, and/or a recent analyst lowering of forward earnings estimates.
So, imo, that's the reason for Zacks' rating. I dont know their success rate so cant advise whether they have merit. I have no opinion on BFDI, but in gerneral, since I only use technicals to determine where I might want to add if a stock I like is selling off; and I look deeper fundamentaly than just chages in earnings estimates; I personaly dont find their upgrades/downgrades useful.