I think I found the answer here: SSL only (don't bind to port 80) - #5 by Whitestrake
I ran Caddy with “-http-port 9080” and that seems to keep Caddy away from port 80. Looks like it has to do with automatic HTTPS. The default challenge for Let’s Encrypt uses port 80 (which my ISP blocks), so Caddy tries to bind that port automatically.
I’m using the DNS challenge, which doesn’t require port 80. So I don’t think this will cause any unforeseen problems.
Thanks for all of the help with my questions. I really appreciate the help.