system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What causes us to have a random "awkward" flashback, even though that flashback had nothing to do with what you were doing?

I'm talking about those flashbacks you have where you cringe just thinking about it.

EDIT: I should note; I don't mean an actual "flashback". More of a bad memory.

EDIT2: You're all awesome. Thanks for the answers.
gpt
Think of our brain as billions of lightbulbs. All thoughts light up places in our brain, and the brightest light is usually what we recognize. Once in a while, mostly when the brightest part is still quite dim (usually when we're doing something repetitive or boring), one of our "background" thoughts manage to create a brighter light than whatever is active. We switch over to that thought as "active", because that's the brightest.