Head gaskets can go for several reasons but by far the most common is overheating of the engine. A block like the lightweight aluminium Suzuki engine can overheat quite easily, and the increased temperature causes increased pressure in the combustion chamber, oil channels and water channels.
Additionally there are many things that can cause the same symptoms.
As posted above rapid cooling (or indeed heating) of the block can cause it to warp. This means that the head and block don't line up any more and the gasket no longer seals the gap. The oil/water/combustion chamber start to mix and everything starts going wrong.
Similarly overheating, rapid cooling or just bad luck can crack the block or head, causing a leak between the channels and the same problems.
Distinguishing between a head gasket failure, a warped block/head or a cracked block/head is a black art and is not wholly reliable.
Grab hold of a Rover 3.5V8. Those things are so big, heavy and overengineered they don't warp, crack or owt.
Of course it'd double the weight of the SJ, losing its primary advantage!
