Typically the issue is steroids allow the muscles to unnaturally grow stronger than what the tendons/ligaments can support which is one of the checks/balances in the body naturally. When you lift weights, the body will never naturally allow muscles to get bigger than what can be supported by the tendons/ligaments. Tendons and ligaments increase in strength much slower than the muscles can because of the relative lack of bloodflow to them. That's why after the initial "jump" observed when starting out lifting weights, the muscle growth tends to slow down, to ensure the tendons/ligaments can keep pace. When this gets overridden by steroids, the tendons/ligaments are no longer able to withstand the additional forces placed on them by stronger contractions than what they can handle and they essentially snap.