Campbell was officially released on July 31 (the first day of training camp) in order to make room on the roster for Isaiah Ekejiuba. Originally Campbell was picked up off waivers after the Giants released him back in June. He is in his first year in the league as an undrafted player out of Minnesota.
This move likely means that Jordon Dizon is headed to the injured reserve. The move hasn't been announced just yet, but I would imagine Campbell will be taking Dizon's spot on the roster.