The charge appears to have been made under a grand jury indictment. I'm speculating here, but I would guess that the DA placed some of the active members before the grand jury, had the court give immunity and forced them to testify against the fraternity's officers. Here's what the DA has to prove:
Tex. Educ. Code Sec. 37.153. ORGANIZATION HAZING OFFENSE. (a) An organization commits an offense if the organization condones or encourages hazing or if an officer or any combination of members, pledges, or alumni of the organization commits or assists in the commission of hazing. * * * [end of citation]
Below is a link to the Texas hazing statutes. The statutory authorization for immunity is included. (This msg is for discussion purposes and is not advice.)
http://stophazing.tamu.edu/rules-and-laws/texas-law