From the team's official press release:
Joseph, a 6'7" forward, was originally drafted by the Celtics with the 51st overall pick in the 2012 NBA Draft. He appeared in 10 games split between Boston and Brooklyn and totaled 11 points and nine rebounds over 54 minutes. Joseph also appeared in 27 games split between the Springfield Armor and the Maine Red Claws of the NBA Development League averaging 18.6 points, 4.6 rebounds, 1.8 assists and 1.7 steals in 34.0 minutes per game and was named to the All-NBDL Second Team. He was acquired by the Celtics in a trade with the Brooklyn Nets on July 12.
After performing well in last year's summer league as a second round pick, he spent some time in the summer league and eventually made his way onto the Brooklyn roster and eventually traded back to Boston. With the team already too deep at the wing position, waiving Joseph and his non-guaranteed contract was the most most logical move as Ainge attempts to trim down the roster.