I managed to create (artificial) reproducible case where scdaemon hangs up.

I have a Gnuk Token inserted, and insert/remove Yubikey for this experiment.

(1) first terminal

$ gpg --card-edit
...
gpg/card> 

This is needed, for another terminal can do DEVINFO --watch for scdaemon by socket connection.

(2) second terminal
Invoke DEVINFO --watch but not consuming its output, by stopping Ctrl-Z

$ gpg-connect-agent
> SCD DEVINFO --watch
...
^Z

(3) third terminal
Observing card statuses.

$ while sleep 0.5; do gpg --card-status all; done

And inserting/removing a Yubikey many times.

Eventually (for my case, 45 times of inserting/removing), the gpg process stops proceeding (because scdaemon hangs up).

(4) fourth terminal
After scdaemon hangs up, listing secret keys hangs.

$ gpg -K