Your brain has layers. Deeper into your brain you find much older structures which deal with fundamental matters like breathing and standing up. Higher up in your brain are more modern structures which deal with more proactive and abstract thought processes. When your brain is stressed it shuts down or reduces thinking in those outermost lobes and retreats into its reptile hindbrain.
Or to put it more simply the part of your brain that controls coming up with and implementing a defence strategy shuts down long before the part that’s just “remember to stand”