Many people believe Brent decided to retire so he could avoid being suspended by the NFL.
According to Ian Rapoport of the NFL Network, Brent likely would have faced a one year suspension by the NFL.
From Total Access: What kind of suspension was Josh Brent likely facing if he hadn't retired? Look at Stallworth years ago. Suspended a year
— Ian Rapoport (@RapSheet) July 18, 2013