This is all speculation but my theory twofold:
1) reduce the number of cars parking on Forbes. They are bumping out the curbs in places so there are fewer parking spaces.
2) traffic calming. By doing this they reduce it to one lane each way, esp. around the big turn at the top. Maybe they'll slow people down.
Of course it does reduce the space for cyclists a bit. I was just glad to discover they were finishing at least part of the repaving yesterday up near the top by Schenley drive.