I saw it in this tanka that I'm trying to translate:
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In this sentence, ゆくらん = ゆく "go" + らん. らん is a later spelling for the auxiliary らむ. It falls under the following definition.
must be —ing because of; why should ... be —ing? (speculating the cause/reason out of the present state of affairs)
Addendum: the translation of so-called 原因推量 usage of らむ could be tricky, because it is used in such a construction:
to mean "V1, [which must be] why V2 now". Thus if V1 is not provided, it becomes "why [have to (end up)] V2 now". For the latter case, a word means "why" can be explicitly added, too.