The answer to 'why is my fence overgrown' is that your fence has likely not been properly maintained and trimmed on a regular basis. Over time, plants, weeds, and vegetation can encroach on and overtake a fence, causing it to become overgrown.
Some common causes of an overgrown fence include lack of regular trimming and maintenance, fast-growing plants or weeds taking over, and not addressing issues like encroaching tree branches or bushes. The symptoms are obvious - the fence will be partially or completely obscured by vegetation growth.