A reason hasn't been given for Rebecca DiPietro's release, although it should be noted that Stephanie McMahon hated her. Even though Rebecca was in a relationship with Batista, she was not his exclusive girlfriend so it wasn't enough to save her.
Joey Mercury's release from WWE this past Monday at the Raw taping in Chicago reportedly had to do with a Wellness Policy violation. This was Mercury's second violation as his first took place in May of 2006, which saw him get taken off WWE TV for six months.
It should be noted that according to WWE's Wellness Policy, a second violation should automatically result in a 60-day suspension with time spent in a rehab center. A third violation results in immediate termination. So, given the policy, Mercury should have only been given a suspension.
However, many feel that following the attention the company got after the big Sports Illustrated article linking wrestlers to steroids and HGH, the company looks to be making a conscious effort to crack down on offenders of the policy. It is worth mentioning as well that when Mercury was seen at Raw on Monday he didn't appear to be clear-headed and was sent home before the show began after he was informed of his release from the company.