Why was the poem entitled "autumn"?

It's because Autumn can have many meanings in literature. It can be seen as maturity and wisdom. It can be seen as old age, but before sickness and death, the harvest of a lifetime of learning. 

It can also mean the coming of the cold, of sickness, of hunger and lean times. In poetry, especially, Spring is associated with youth and new, budding life. Summer is the pinnacle of life, when you are older, but still young enough to have family, success, etc. Autumn is often the time of retirement, but you have a sense of fulfillment, richness, wisdom, freedom. Winter is old age, illness, the death of loved ones and, ultimately your own death.