Infected Brute glitch
From Halopedia, the Halo wiki
This is a software glitch on Halo 2. This can cause some Brutes to rise after being killed. Usually, they stay in their battle ready stance they perform when ready to run. The software thinks they are in "death" mode, but they are standing still, but they won't move.
Due to one of their sightings being in the first part of the level High Charity, where the Flood are all over the structure, it is thought of as them to be infected.