Novelty/troll accounts. I’ve seen a lot of them pop up on Reddit over the years. The most infamous one I remember was one called Gradual_****** (yes, the n-word) that would type out ordinary replies that would gradually switch to ebonics.
Reasons that are fine to mention: Last job was temporary, I was made redundant.
Reasons that aren’t fine to mention: I quit due to overwhelming stress, I was fired for capability/gross misconduct reasons (unless it was an unfair dismissal which you successfully challenged in court and have the documentation to back it up.)
Again, depends on why. If there’s documented evidence that you were fired for say… refusing to ignore safety laws.