The area between the valves compresses when hot and open up when cold.

The hotter the engine, the greater the compression on the thin metal between the valves.

Another factor is how hot your exhaust gases are. Retarded timing, poor fuel mixture bring up the exhaust gas temp and dump a lot more heat into the head.

Basically, a poor running motor can shorten the life of the head.

My experience is similar, they all crack. Its all about the number of heat cycles and how extreme that determine how long it it before the crack reaches the water jacket.

Most heads have a hairline crack by 60,000 miles. The 2.6 is not the only motor that does this.