Why does it rain cats and dogs in the UK? Why do Americans have ball-park figures? Don't think too much about it. For the origins of the expression, see other comments in this discussion. Not that they explain the moose. The moose are an ineffable sort doing ineffable things. ;)
Right. I concede that the musicians' slang 'Groove' appears as verb. but only as a participle, in a progressive tense, usually if not always with a human subject. You can't say something grooves. You can say it swings. But the moose is really far out, even for jazz argot.