An engine block – like any piece of cast iron – can crack for no apparent reason. Not a common thing fortunately. The block starts out as a mold, then molten metal is poured in, and allowed to cool. If the molten metal is contaminated, or it doesn’t cool off slowly enough or unevenly, stresses can develop from which a crack can form, sometimes much later. After the casting is cooled and removed from the mold it then undergoes a lot of machining, drilling, etc, and if it is too roughly handled that can induce cracks.
If you have a chance to see it, sometimes PBS airs a show about a company that manufactures cast iron pots and pans. At one point they show a huge pile – like 20 feet tall – of them sitting outside, waiting to be remelted and cast as new. They said some in that pile were from defects pulled from the production line, but others were returns from people who bought them, then later they cracked and leaked, so were returned to the company for replacement.