Something both fighting (aggression) and frighting (panic) have in common is that they involve adrenaline rushes. Adrenaline rushes make people emotionally volatile and act unstable.
It is also not uncommon for victims of domestic violence to feel angry with the abuser and to take revenge when it is safe to do so. The only time it is safe to do so is when there are witnesses around.
So someone who is being abused behind closed doors can seem really unpleasant in public, while the abuser seems meek and mild.
Another common tactic by abusers is to subtly bait the victim in public until the victim loses their temper and verbally erupts. It's a form of gaslighting the victim, making the victim seem like the crazy one and undermining other people's opinion of the victim.