The more defective your cells DNA copy gets the more irregularities it will have and will then pass that copy of bad DNA onto other cells when it duplicates. This is the same reason moles and freckles start appearing on your skin, and why long hairs are more likely to occur on freckles and moles. By just missing one letter, or mixing up two letters it could completely change the function of the cell or change key characteristics of the cell-like the max length your hairs are suppose to grow.

No matter how good you are at typing, you still mix up a letter or two once in a while. Every time a new cell is made in your body the DNA gets duplicated, but it is more like typing it manually than just making a photocopy. Similar to how a computer uses binary to tell a computer how to run (010100100111 etc) DNA uses four different nucleotides (ATCG) and depending on the order of these 3 billion nucleotides different instructions are given. Your DNA consists of over 3 billion small units called nucleotides. These instructions are stored in something called DNA. The ELI10 answer: Every cell in your body has a set of instructions that tells your cells how to behave, including how long your hair should grow before replacing it with a new hair. The ELI5 answer: They are caused by a mutation in your genes.