I believe that for many years, MLB managers would always wear the same uniform as their players during games. However, I've been watching more games on TV this year, and it seems that many managers now wear other clothing.
For example, in this photo from May 9, 2015 (credit David Richard of USA Today), we see Cleveland manager Terry Francona making a visit to the mound. The shirt he's wearing is completely different from his player's uniforms, and it doesn't look like he has a uniform jersey underneath. Why the break with tradition? Was there a rule change, or simply managers deciding to dress differently for some reason?