Place X has a famous professor you absolutely have to work with
Dig out on the Internet some ancient rankings of graduate departments (possibly scanned from clay tablets), and don't check what methodology was actually used to make these rankings, or if these rankings have anything to do with the strength of the graduate program and the probability of your success;
Your mother told you to choose Z;
Ask for advice on the Internet from a bunch of people you have never met, and whose real names you don't even know;
You want to spend a lot of time surfing (or skiing or rock climbing) while being a graduate student;
The department was a bit old, but the weather was really good when you visited.
Adapted from http://www.astronomy.ohio-state.edu/~kstanek/howgrad.html