Infected Brute glitch
From Halopedia, the Halo wiki
A software glitch on Halo 2. This can cause some Brutes to rise. Usualy they stay in their battle ready stance they perform when ready to run. The software thinks they are in "death" mode, but their standing is still there.
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 thaught of as them to be infected.