Writers deliberately plant a seed for specific fanon to develop.
This is when writers want to include an idea--any element contributing to the plot or a character--in canon, but don't want to explicitely state it. As a work-around, the writers hint at the idea until it's accepted by most of the fandom. These hints are usually strong enough that most of the fandom gets the right idea fairly quickly, but not so strong that they can't be ignored or attributed to something else if the viewer dislikes the idea being hinted at. This tactic is most often employed when writers want to include an element such as mental illness, rape, or another sensitive topic in a plotline or a character's backstory as a means of plot or character development, but don't want to explicitly state it to avoid controversy or alienating certain viewers. If the hints are particularly weak or ambiguous, it can be difficult to determine if they are deliberate or merely a coincidence. Short of a Word of God confirmation of intent, the easiest way to establish this is if the idea is built on as if it was canon. This trope is most common in live TV-shows, but shows up in other mediums fairly often as well.