In this sentence, = "go" \+ . is a later spelling for the auxiliary . It falls under the following definition.
> ② ……
> _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 usage of could be tricky, because it is used in such a construction:
> V1(-continuation)V2
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.