Dear Jack
Two points here:
First, I have recently been persuaded that I should really use the term "top down realisation" rather than "top down causation". Causation happens at each level of the hierarchy of structure and causation rather than up or down, whereas realisation is the downward relation and emergence is the upward relation.
That being said, causation does indeed happen at each level at the same time: in the brain for instance, 1. electrons flow in axons according to Maxwell's equations, 2. genes are turned on and off according to the state of gene regulatory networks, 3. information flows down axons to synapses where signals are summed and used to decide if new spike trains should be sent on, 4. pattern recognition takes place via the neural network structures formed by the neurons, and 5. logical thought takes place at the psychological level, for example deciding if Maxwell's equations are a good description of electromagnetism. The lower levels enable the higher levels to function, but it is the logical highest level that decides what the final result is (Maxwell's equations get written down as a good description of reality)
Second, "causation itself a singular event/operation that necessarily requires a singular explanation". No, there are multiple causal relations in action at the same time, as indicated above, and as point out by Aristotle in the Wikipedia article on Four Causes. I elaborate further in my book on top down causation, taking the example of why an aircraft flies. It flies 1. because of air particles impinging on the wing, 2. because the pilot is flying it, 3. because it was designed to fly, and 4. because someone is making a profit out of its flying. These four causes operate simultaneously; and without any one of them, it would not fly. Physics is concerned only with level 1. The other three levels are also essential.