Someone says something to us, and we are suddenly struck with a sinking feeling in our stomach. Someone does something, and instantly we become enraged or alarmed. Someone comes at us with a certain attitude, and we go to pieces. We hear mention of a person, place, or thing that is associated with an unresolved issue or a past trauma, and we immediately feel ourselves seize up with sadness, anger, fear, or shame.
Tunnel vision happens when your mind zooms in on a single "threat cue" and filters out everything else. In this case, the threat cue might be: "He was young." "It was cancer." "It seemed sudden." "He probably didn't see it coming." Your mind grabs onto these details and begins building a narrative: "Cancer is everywhere." "People are dying young all the time." "It's inevitable that I'll get something serious." "If I do get sick, there will be nothing I can do."