It's a combination of too small of a tank and lack of targets to channel their aggression towards.
I believe it was duanes that saw Jack Dempsey pair in the wild guarding a 10'x10' ish area when breeding in the Cenotes. Even assuming a tank with just a foot of height, that's a 745 gallon aquarium for a fish that might max 10".
Add to that these fish do not stay together outside of the breeding period (most new world species, there are exceptions of course) and would normally try to kill each other ... they need other fish to turn that aggression towards, and in a 10 ft area, there would be plenty of targets swimming in and out of to keep the parents busy all day long.
We, however, usually don't keep breeding pairs in tanks with other fish, because we keep them in tanks that are too small so they normally would kill their tank mates, and without that threat they turn that aggression on each other. Or one (usually the male, but can be the female as well) is ready to breed again but the other is still defending the fry. In nature the reluctant one would just swim off, but again, our tanks are too small to allow this.
