How do people get sarcoma cancer?

We don't really know: Most sarcomas are sporadic, and they don't (usually) run in families. Some very rare genetic disorders may predispose subjects to them. Other rare causes are radiation therapy use (in very few breast cancer survivors), exposure to an old contrast agent called thorotrast, chronic lymphedema, and HIV (kaposi sarcoma).

Usually random: Sarcomas are bone and soft tissue cancers. Generally, these occur because of random events in most patients. However things like radiation exposure can increase the risk of bone and soft tissue sarcomas particularly with higher dose radiation like used for treatment of other cancers. There are some genetic diseases like li-fraumeni that can result in getting a sarcoma and other cancers.