Sometimes people feel it's safer to ride the wrong way in the bike lane than to cross a busy street to the opposite side. I've seen that a fair amount from the BlueBike corral on Garden St; the Harvard students feel it's easier to hop onto outbound bike lane to get them inbound towards Cambridge Common (rather than cross Garden, bike down two blocks, and then cross again towards the Common).

We'll see if that changes now after the new traffic routing in that section, which should include a safer story for cyclists heading towards the Common from SE-bound Garden.