Brain injury causes problems with anxiety two ways. The most common is from a failure of the part of the brain that filters stimuli. 90% of the stimuli we experience is usually filtered out as not meaningful to the tasks at hand. Visual, auditory, olfactory (smell), even tactile stimuli all combine to overwhelm the brain's ability to process once they sneak past this filtering system. You could compare this to a kitchen faucet if you took the aerator off. That previous soft restricted flow of water now comes gushing out splashing all over.

The over-stimulation is anxiety. The brain tries to handle it and spaces out in confusion. Everything just wants to spin. Anxiety.

The second way is over expression of adrenaline. This can be due to a physical injury to the pituitary system and surrounding structures. This can kick the brain into overdrive speed. It can cause outbursts and other troubling behavior. Blood pressure can spike. Just a mess. This is the basis for the fight or flight response.
It is not as common as the first way.