There are a lot of factors or reasons X-Plane could crash. It could be from addons like plugins, scenery or third party aircraft. The computer could be overloaded. It could be a legitimate bug we haven't found good reproduction steps for yet.
The first steps to investigate are looking at the log.txt from immediately after the crash, as well as the Apple crash reports which are found in ~/Library/Logs/DiagnosticReports/ (where ~ refers to your Home directory). You may have to show hidden folders to access them.