According to profootballtalk.com:
After toiling as the third QB in 2006, Shockley blew out his knee before the 2007 season and missed that year. He beat out Joey Harrington for the No. 3 spot last year but John Parker Wilson, an undrafted rookie from Alabama, beat Shockley out. Given the extended play Wilson got last Thursday when he was the second man in after starter Matt Ryan, Shockley's release was looming.
Atlanta needs to lop a dozen more players to get to 53."