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". The "why" part can be explicitly added, too.