R.T.F.M.
I don’t know how many years I’ve used baking powder without noticing the notice to “shake well before opening.” I guess that makes sense, though, that perhaps the leavener and the acid and the anti-caking agent might settle into different strata while the jar sits on your cupboard between uses. Shaking it up would get them all evenly mixed again. Amazing how often you can use something without really looking at it.