The former Buff was the first to commit in the 1995 recruiting class under then-first-year head coach Rick Neuheisel. He lettered four seasons, from 1995-98, and had 167 career tackles.
Current CU inside linebackers coach Brian Cabral mentored Southward at CU.
"This is a shock, just devastating," Cabral said Thursday in the CU SID report. "(Brandon) was happy go lucky, a free spirit in a good way. He took great care of his body, was well conditioned, did everything we asked of him, and he just loved every part of the game. He was a pleasure to coach.
"I spend a lot of time with my guys, and they're like sons to me. This is a tough loss."
Funeral arrangements were not determined as of Thursday afternoon.