"Raising a flag" means making a statement or taking an action that, according to narrative convention, almost guarantees a specific (and usually opposite or unfortunate) outcome. It's the concept of "jinxing it" or "tempting fate," but with a stronger connection to predictable plot devices in fiction.
The Origin
The "flag" is a metaphor for a plot flag or a cliché signal. Writers use these "flags" to set up future events. When a character "raises a flag," they are unknowingly activating one of these well-worn narrative traps. The term gained widespread use through discussions and reviews of anime, manga, and later, Chinese web novels.
The Deeper Meaning
"Raising a flag" is essentially Murphy's Law ("Anything that can go wrong will go wrong") codified as a storytelling rule. It creates dramatic irony, where the audience knows more than the character about their own impending fate.
The phrase "The more you don't want it, the more it will happen" is the golden rule of flag-raising. By verbally acknowledging and fearing a negative outcome, a character is, in the logic of stories, amplifying its likelihood to near-certainty.