Now imagine the water is suddenly turned into vegetable oil (more load has come on). It is more difficult to move. For a brief moment, your energy input is still the same but you can't push as much of the new liquid so your pace slows down a bit, to 58 steps per minute. You increase your exertion (add more energy input) and your pace climbs back to 60 steps per minute.
Finally, imagine that you're running as hard as you can. That 60 steps per second to push that vegetable oil is as much as you can do. But the vegetable oil turns to heavy cream (even more load). You've put in as much energy as you can but your pace falters and drops to 49 steps per second.
That's why the frequency falls.