Most of the heat in the human body is generated within mitochondria ... what is occuring in them and why does it produce heat?

Mitochondria are the organelles that prevails within some of the eukaryotic cells. The humans are endotherms that is they maintain the temperature of the body within a stable, narrow range by internal production of heat and monitored heat dissipation.

The endotherms depends in metabolism or production of energy inside the cells. Mitochondria generate the majority of human body heat as they monitor metabolism by controlling the majority of intracellular production of energy.

Mitochondria mediate metabolic reactions, and in any reaction energy is transformed; some is lost as heat.