Gotta be more to this DUI we don't know about. It was dismissed, as far as I know TJ has not had any previous issues, and half a season heavier than many previous DUI suspensions.
He must have had a failed test for substance abuse in his past but the first time failing isn't a suspension and can go under the radar/withheld from the media etc.
I'm guessing since his first failed substance abuse test was already in his back pocket, the current DUI wasn't alcohol related (likely a controlled substance of some kind) so he graduated to the stage 2 penalty phase and you combine that with being behind the wheel of a vehicle, probably got him an extra couple games added to his suspension. (I think something to this extent)
http://www.baltimorebeatdown.com/2016/3/17/11250306/a-refresher-course-on-the-nfls-drug-policy
A player fails the test if their sample tests positive for illegal substances (obviously) or if they fail to show up to the test. Failing a drug test moves a player to Stage One of the NFL's Intervention Program. Stage One is simple, the medical director evaluates a player and then comes up with a treatment plan for him at a facility or through a clinician.
The player is subject to substance abuse testing, but it is not yet at surprise at this point. Stage One can last up to 90 days. Violations of Stage One only result in a penalty of a fine. If a player successfully completes Stage One,
they are free, but they enter Stage Two if they fail.
Stage Two is similar to Stage One in terms of treatment, testing, and violation. However it lasts for 24 months instead of 90 days and players that commit infractions
can face suspensions of four to six games. Failures here send a player to Stage Three of the NFL's Intervention Program, where Will Hill and Martavis Bryant reside.
Like with the other two stages, players are subject to a treatment plan, but they are now subject to unannounced testing. Failed tests will result in a 10 game suspension if the first violation is for marijuana. Unless the medical director chooses to discharge the player, Stage Three will last for the remainder of the player's career.