some articles out there on this. Found one written by CNN.
They report that Brown was involved in a shooting last year, a guy died, and that case has yet to go to trial. The accused is out on bail. Brown, shot in the incident, would be a key witness. Brown believed he was the actual target of the shooting. Brown feared for his life and was trying to keep a very low profile, which is why he didn't want to participate in the Guyger trial.
In addition, the summary of Brown's testimony, as provided by CNN, sounds like he corroborated Guyger's version of events. The Dallas PD is mad at Brown for testifying that what Guyger said happened.... happened?