> Why wouldn't the change in day length be the initial trigger? 

Because long-lived bees appear under a wide variety of circumstances, not just shortening days. As has been mentioned, queenless bees, bees preparing to swarm, probably bees preparing to migrate, bees during periods of drought, etc. exhibit lengthened lifespan, and this apparently is a result of sequestering lipoproteins. 

Apparently, these bees are present in colonies at all times. The question is: what is the trigger that causes some bees to stop sequestering, which leads them to the path of foraging and drastically reduced lifespan. 

The fact that differences between temporal castes often
override the effect of age on damage suggests that functional
decline and mortality are not gradually increasing
but rather are caused by a reallocation of investment in
the short-lived caste. When nurses turn into foragers, the
bees lose weight ... Weight loss could be a symptom
of divestment of resources in individuals experiencing
high extrinsic mortality. Regardless of the
reason for the weight loss, it may deplete reserves that
otherwise could protect tissues from damage.

de Verges J, Nehring V: A critical look at proximate causes of social insect senescence: damage accumulation or hyperfunction?, Curr Opin Insect Sci (2016). Article in press

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