Despite being bizarre - and even a little creepy (don't get me wrong, I usually like creepy stuff, but not in this case) - having horns is not so unusual for humans around the world. The explanation for them is actually very simple: the horns are originated by skin lesions that cause keratin (the protein that constitutes nails and hair) to accumulate in a certain spot. That spot can be the head, scalp (the most common), hand, ears, lips, nose and even in the eyelids!
The lesions can have benign or malignant features, but scientists say that most are benign (about 60%), although the rest of them may develop cancer cells. Sun exposure can worsen the problem. The horns can be taken off by surgery but, in certain cultures - Asia, I'm talking to you! -, they're often allowed to grow when they are not cancerous.
Human horns are not like animal horns though. "Ours" are just stuck to the skin, while bulls, buffalos, etc., have their horns as skull attachments.