Number one cause of head gasket failure is overheating.

Failure is more likely for an engine that is heated and cooled frequently - e.g., started, stopped many times a day. The reason is the uneven expansion of the aluminum head and the cast iron block. Aluminum expands more quickly, causing the gasket to grate against the more stable block. More grating related to more frequent heating and cooling cycles will wear the gasket down.

The more wear on the gasket, the more susceptible it is to failing, even with minor overheating. On an older engine with original gasket, a blown radiator hose and resulting overheat can blow the gasket and warp the aluminum head in a matter of minutes - by the time you notice the overheat, it's already to late to prevent the damage.