Should I explain a single bad grade in my personal statement?
There are really two parallel circumstances to consider here:


The grade is highly relevant to the program in question. In that case, you might need to address it, as someone will be focusing on that coursework in particular - partially because they will be relying on you knowing the content of that coursework, and with that grade it's hard to argue you do. Though in this case, it's also probably a good idea to strongly consider retaking the course. In this case, you might touch on the reasons, but that should be 1) Short and 2) Be a legitimate reason why this might not accurately reflect your abilities. "I didn't like the professor" is not that reason.
It's just a random class. 'Women in Medieval Art and Literature' and you're applying to a Microbiology program. 'Calculus 1' and you're applying to a Comparative Literature program. 'German 4' and you're applying for a Computer Science program. In this case, I wouldn't draw attention to it.