Dangers and threats, duty and responsibility, loss of control, losing an advantage

A police officer losing their firearm in a fight with a suspect
Having to fight off a kidnapper without a weapon
Being attacked by a bear or cougar while hiking
Being attacked by a shark during a dive
Being assaulted in a dark alley 
A weaponless character facing down an armed home intruder
Being mugged at gunpoint
The character facing an assault while they're vulnerable (in the hospital following surgery, at home with a broken leg, while they're intoxicated, etc.)
A mild confrontation turning violent when the other party pulls a knife or gun

Being caught off guard and so even more vulnerable
Clothing choices that put the character at a disadvantage (having to run in flipflops, the attack happening while they're in their pajamas or naked, etc.) 
Not being able to pinpoint the attacker's location (because it's dark, the attacker is hiding, etc.)
Having to find a place to hide
The character struggling to silence their own rushed breathing so they can hear or hide
Needing to stay still in an uncomfortable position
Being with someone who is likely to make the situation worse (because they're impulsive, confrontational, macho, or foolish)
Needing to stay calm in a high-emotion situation
Being responsible for someone (a child or elderly parent) and needing to protect them, too
Being alone with no hope of aid from anyone else
Sustaining minor injuries in the attack (cuts, scrapes, a black eye, etc.)