The answer to 'why is my fence overgrown' is that your fence has likely become overgrown with weeds, vines, or other vegetation. This can happen over time as plants encroach on the fence line, especially if the area around the fence is not regularly maintained.
Some common causes of an overgrown fence include lack of trimming or weeding, poor soil conditions that allow weeds to thrive, and nearby plants or trees that spread and grow over the fence. If left unchecked, an overgrown fence can become unsightly, make the fence less effective, and even potentially damage the fence structure itself.