If you're playing on Beginner, Normal or Hard difficulties, Scrakes will rage when they've lost 50% of their health. The amount of damage needed to trigger a Scrakes's rage changes depending on the difficulty you're playing on.
They move so fast when raged that no perk can outrun them so once a Scrake is raged, you have no other choice but to face it down and kill it. Once Scrakes have taken enough damage, they will go into a raged state where they move much faster and they will not stop raging until they are dead.
