They explode, but why? High temperature and pressure are two of the most important elements. Inside the maize grain, there's humidity and then it's contained with the exterior shell. During heating, under a temperature of about 150 – 175 degrees Celsius the vapor tends to expand creating considerably high pressures near 10 bars!
It's really interesting how popcorn don't explode simultaneously creating great noise and a bizarre experience.
After, the pressure breaks the exterior shell producing a fast expansion of the genetic material producing a marvelous texture, appreciated also for its taste and smell. According to several sources, if some grain doesn't explode it would have relationship with the inability of the kernel to produce pressure due to eventual damage in the shell or scarce quantity of promoter. Both factors that converge with handling and production. Of course, it would be more explanations at the stochastic level via the genome.