What we are dealing with on the quarters issue are two (1) unlikely events that (2) occurred simultaneously with a (3) shared theme.
Joker lacks the quality of being unlikely. There are tons of superhero and antisuperhero movies drawn from comic books. And there are several movies about misunderstood isolates who spawn social unrest, V for Vendetta and Fight Club being just two that I can think of.
Further, there is no shared theme between The Joker and bat flu. We might say that they both involve Batman, but only as something that is related to Joker and never even appears in the movie.
When we allow one of the events to be only related to the theme of the other, we raise the possible events that can satisfy the shared theme criterion and thus drastically raise the probability of the concurrence, thus reducing the need for an explanation.
What happened here is not that something related to bats was on the quarters. I was very specifically bats.
Furthermore, there is another shared theme between the bat quarters and bat flu. Not only do they both involve bats, but they are also both tied in with the economic system. The biggest fallout from bat flu could be economic, and the bats aren't in a movie or in a song or something unrelated to bat flu. They are on the money.