According to the officers, they noticed immediately that prisoners in the yard were acting different from the usual from differences in the way they went about their usual routines. They also noticed the extra layers of clothing on two groups of prisoners despite the warm weather, which immediately suggested to them the potential for imminent violence. Since only two groups with known pre-existing animosity were overdressed, the officers believed they were facing imminent gang violence and they followed institutional protocol for returning inmates to their cells. It was apparently preplanned that the attack on the guards came from those two gangs in that process. Inmate's Recollection of a Riot:

One inmate advised that "not only can't you trust anyone in here, but you also can't trust anything."...

Because of their history, the corrections officers working the yard immediately assumed that the signs of imminent violence would be between the two gangs (the Northern Mexicans and the Aryan Brotherhood).
The protocols used for preventing gang violence in the yard were used by the inmates against the guards. By design, each gang attacked and overpowered the group of officers assigned to escort their members back into the housing facility. By feigning the indications of gang violence, the two gangs succeeded in halving the number of guards available to respond to each separate coordinated attack.