The Eagles were likely being reviewed due to the fact Mathis was released after voluntary OTA practices ended last week. Some may theorized that Mathis was being punished for not attending those sessions, but clearly that wasn't the issue. According to a report from Albert Breer, the NFLPA has cleared the Eagles' decision to release Mathis.
"The NFLPA saw no wrongdoing on the Eagles' part in releasing Evan Mathis. Seen as a contractual issue. They've been cleared. [...] The union sort of saw it as a non-story all along. Again, no wrongdoing."
One of the reasons contributing to the release of Mathis was that he's been wanting a raise since 2014. During the 2015 NFL Draft, Chip Kelly said the Eagles have made Mathis available for trade but no one had made any offers for him.
The 33-year-old Mathis is currently a free agent able to sign with any team. It'll be interesting to see where he lands.