I have a dozen or so ball caps that I keep in the garage. Either they
have all shrunk or my head has grown, and I don't think my head has
grown (at least literally), nor am I wearing my hair longer. Included
in that number are several fitted caps that are way too tight now to
wear. Before I throw out all these caps, and certainly before I buy a
new one, tell me -- (1) why are my caps shrinking?, (2) can they be
expanded to fit my head once more, and (3) how can I avoid this same
outcome in the future?

I have no clue about the shrinkage, other than to speculate that high
humidity in the garage might have caused the fabric to shrink.

On this page are a few cap-storing and cap-shaping devices that might
help:

http://216.71.213.241/cgi-bin/capgen.pl?cat=AC

I am betting that your head actually did grow.  Check the size of your
fitted caps, and go get fitted for a new one.  Compare the sizes, and
that will tell you your answer conclusively.

You're right, my head has grown 2 sizes (1/4 inch). Too much trivia in there.