When they become more frequent than normal. I work near the Houston ship channel. I work in the oil/gas/petrochem industry.
This may seem an unusual rate to you. May I politely suggest that is due more to the your past awareness, than any change in rate of explosions.
Not that any of the conspiracy freaks on this thread will care, but the explosion locally was said to have been in a grain bin. Grain dust (corn dust especially) is highly explosive, but the MSM could hardly pass up the opportunity to blame this accident on something that apparently wasn’t involved.