I thought it meant to swarm around in a crowd, like chickens in a farmyard. (From pullulus, a chick, akin to English “pullet”.) But it doesn't. It means to teem or swarm abundantly, from pullulāre, to sprout or send forth new growth.
(Which is also akin to pullulus and “pullet”.)
I would like a word something like “mill” in the sense of
but with a connotation of a crowd of chickens, or perhaps a swam of insects, instead of the systematic circular motion of a turning millstone.