I'd noticed it starting Monday, and assumed it is a slow zone due to current or impending track work.

As for the slow zone as one exits or enters Harvard to or from Central, the legend(s) I'd (variously) heard was(were) that the tracks pass beneath a library/residence hall/something on campus and the noise/vibration disturbs sleep/study/whatever, so Harvard prevailed on the T to put in a slow zone. This could, however, be utter crapola.