A Possible Explanation: I believe the metaphor at the end, "being men as well as leaves, to die for the sun", is referring back to the personification/simile seen here: "midsummer's leaves race to extinction like the roar
of a Brixton riot tunneled by water hoses".
In tandem, the word lemmings has two meanings, as read in the dictionary: "a small, short-tailed, thickset rodent related to the voles, found in the Arctic tundra."
and
"a person who unthinkingly joins a mass movement, especially a headlong rush to destruction."
I believe "autumn's fire" and "the sun" are both symbolic of war, an idea of it, and/or passion for it. I read that this poem is criticizing the human aspect that is war and how short-lived and ignorant it all ends up coming out to be.
I hope this helps ♡
