"Bull" is being used as a double entendre -- meaning both the animal shown on the
nickel and the other meaning of stupid or untrue talk or writing; nonsense. So, the bull is coming off the
nickel and being applied to the situation at hand. The situation at hand being nonsensical.
This obscure phrase is famously used in the
Twilight Zone episode "Mr. Garrity and the Graves". Rod Serling, in the intro, uses this expression in referring to Jared Garrity, who makes a living restoring life to the dead.