I agree with other posters that the most important factor is probably self-selection.
I also understand that the school you mention specifically, WWS, focuses a good bit of its mission and unique scholarship program on providing a low cost degree to people who want to pursue work in the non-profit/local government sectors in careers that generally don’t pay well and where graduating with a lot of debt would be problematic for them. Therefore, although there wouldn't be an automatic ideological slant to those candidates and positions, it is probably true that both the applicants and many of the institutions that are offering those types of jobs, trend towards liberal/left. As a result applicants that the fulfill WWS’s profile of demonstrating a plan to pursue a public policy career in low paying fields, would be somewhat more likely to describe a career in a liberal institution. That said, I think WWS is kind of an outlier, and probably not the best place to get a feel for the overall dynamic of most MPA programs.