Why do sudden fear comes in our mind with any automatic thought and unimportant issue?

It's how the fight of flight reflex works, it's the nature of anxiety. It's a panicked, feeling unsafe state. Your brain is trying to keep you safe, but is so ungrounded and waked out that it sees everything as a danger.

Sudden fear may be your brains survival instincts kicking in, it may be confused and not understand danger.

Sudden fear comes out without a warning of automatic thought or important issue reminder is because when you're experiencing it many times you're mind doesn't have time to tell you it and instead it directly sees danger and acts without a second thought.

Our mind created problems that will never exist.Automatically we are afraid of the consequences of a minor thing.

Everyone is different. If you feel you are having an unexplained episode or episodes of fear, perhaps writing those fears down or journaling them can help you. Also you could try speaking with a trusted person or counselor or therapist to help you sort out your feelings.

We evolved to survive dangerous situations. Fear is the emotion that empowers us both mentally and physically to do this. There are three possible reactions: Flight, Fight or Freeze. Our neocortex is involved in conscious thinking. This is the newest part of our brain in regards to evolution. It 'feeds' in to the more ancient parts of our mind involved in more basic emotions like fear. Apologies for the simplistic response... if you want a detailed answer ask a neurologist or evolutionary biologist or a behavioural psychologist.

Things that are in your subconscious may be suddenly brought to the forefront by seemingly unrelated triggers, e.g. seeing a dog with a ball may remind you of your father, who always used to love going to the park with his dog.

Because people oevrthink, some do it more, some do it less. So problems seems bigger than they are but venting everything out can help us see that nothing it's so tragic as we see it. For people who overthink much it's hard to stop it but remember you can do it what you want and I used to overthink too so I tried a little exercise to stop soing it and it worked, so maybe it can help. All you got to do when you start overthinking is stop what you're doing, breath and then shout really loud in your head: STOP!. Then continue what you were doing when you feel ready. It's difficult at first but it should get better once by once. You can do it. I believe in you!