Mine was caused by trauma to the skin.
One day my lips were dry and parts of the skin were sticking out. It bothered me so just bit the skin off and soon I found myself biting off all skin of my bottom lip. The next day I did the same thing and developed a habit of lip picking. I did that for a year and after that it just went downhill from there. Now it's 5 years later.
I don't think mine is a fungus.