What is the medical explanation for low heart rate during exercise?

Several. There are several possibilities. If you're a seasoned athlete, it's due to improved stroke volume (blood ejected with each contraction) requiring a reduced heart rate to achieve a given cardiac output (the product of sv and hr). Other possibilities are: medications, vagal reactions, chronotropic incompetence, av nodal block, or sick sinus syndrome. Incidentally, typhoid fever can cause it too.