Why do I sometimes feel so anxious even when I'm not in a triggering situation?

It could be different reasons. In my case I feel anxious when I'm not doing anything or when I'm bored. Try to be always active.

Anxiety doesn't go away. You can always dull it, but it is always present. This isn't bad, it isn't unrelenting, it just becomes a fact of life that when you're stressed, you'll be REALLY stressed. Though that anxious feeling will never completely die, you can help cool it down with exposure. Every time you march on through that nervousness, it will become a little bit easier.

Because stressing over it causes anxiety so don't stress and think positively towards the situation because a positive mind leads to a positive life

Anxious is all about a pinch , a guilt, a hidden dark side which have to be taken care off,because it mostly get reflected in unwanted situations.

I do feel like this before. I was confused and really don't know why that happened. But as I think about it, I recall all the past situations I've been through and all those situations, I was stressed out but forcing myself to say 'it'll get better'. I try to avoid the anxious feelings and repress in deep down in my mind. And for a few days, while trying to be calm, I suddenly felt being really anxious without seemingly triggering factors involved. In fact, it was a positive situation I am in. However, the positive situation I am in was once the cause of my stress, so perhaps, it brought up the repressed anxieties I have before. However, this is just my case and things might vary on people. It's vital to look at the situation you are in and take note of the circumstances where you feel anxious without the triggers. This way, it will somehow help you understand your emotions. It is also important to take control of it. And if really bothered about it and don't know the reason why, seek professional help.

The anxiety could be due to the fear of a triggering situation arising and you are unsure what to do when it happens.

Everyone is different. Its not a one answer fits all. If there is a feeling of anxiety there often even when there are no triggering situations, perhaps it may be time to speak with a trained professional to help with your feelings.

Sometimes my surroundings are causing subconscious anxieties. I've noticed that for me this happens when I'm in a crowded room

Because you're worried of the things that could happen, or could never happen. You start to think too much of the future, even when you should be enjoying the present.

Due to lack of sleep mostly. I'll feel anxious for days and not really know why. My body has a mind of its own.