Because he was thrown overboard.....if you're a cop and your department isn't 100% behind you, you'd better find a different department to work for....you aren't safe there.
He has barely had time to speak to his union, he didn’t have to resign, unless he felt that he just wasn’t cut out for this work.
Why did he leave the Texas Highway patrol after such a short time?
Resigning instantly like this, doesn’t look like the way to vindicate himself for remaining in police work.