Load on the engine means slow engine speed and lots of throttle opening. The cylinders are getting full charges of air and are under lots of pressure. That pressure needs a strong spark to ignite, and if the system is weak, it's easier to "jump the gap" outside of the cylinder and you get a miss.

I had a coil break down on me when it was hot. Left me stranded. It started by backfiring and popping under load, but would rev fine in neutral. Got so bad the engine finally stalled and wouldn't restart. After it cooled down it ran fine again.

I replaced the coil and it never happened again.

--it was also that experience that caused me to think it was my ignition system again when my booster blow out, and why I kept driving it trying to figure out what was wrong with my ignition, before thinking that maybe it was a vacuum leak instead.