why were there so many acorns in 2020?

The fall of 2020 was part of what's called a mast year, which occurs every 5 to 10 years. During a mast year, oaks produce an extra large amount amount of acorns.

Why would oaks have evolved to occasionally use up extra energy to produce an excess amount of nuts? The extra seeds during a mast year ensure that there are enough for the tree to propagate.

The animal population that consumes and stores the nuts, in this case primarily squirrels; is balanced based on the amount of acorn food source that can support that population. During normal years, a balance of predators and food source becomes established, because if there is not adequate source of acorns the squirrels in that location either move to a different location or their population thins out due to malnutrition. Since size of the animal population has balanced out to consume all available seeds, in a normal year there may not be adequate seeds leftover for the trees to reproduce. The abundance of acorns during a mast year ensures that some will be left over to propagate and grow into new trees.

Note that there is a different situation where you see just a single tree (not all of the trees of a single species in an area) producing lots of seeds. This excess production of seeds can be a sign that that one tree is declining or stressed. A tree that is declining may be producing extra seeds to ensure that a new seedling will grow.