Causes of a bad credit score include, taking out too many loans in a short time period resulting in too many hard inquiries on your credit report which lowers your credit score.
Other things that can cause a bad credit rating are not paying credit cards on time, defaulting on credit cards and defaulting on loans, repossessions, defaulting on a mortgage, or car loan, and even not paying utility bills on time or having utilities shut off and not paying the amount you owe to the utility company can result in a bad credit score.
Even cell phone bills not paid on time reflect on your credit score and can lower your credit rating.