I contacted customer service and got in part the following reply:
--
Please know that if your email account sends 50 or more messages to
the same group within 20 minutes, Yahoo! Groups will suspend your
account with the status of Auto-responder. Unusually high message
volume is often an indication of an email program set to auto-reply
with an "I'm-on-vacation" type of message.

As long as your status is Auto-responder, you cannot send or receive
messages. You can prevent this by setting all of your group delivery
options to "No Mail/Web Only" before you go on vacation.
--
I don't have auto-response enabled anywhere and I can't find "No
Mail/Web Only" anywhere.

Hopefully this was a one-time event because I don't know what
happened.