Is there an added message with the bounced messages? If so, what does it say?
Remember that Yahoo groups have a restricted size for their archives and their mailboxes (20MB, I think). It may be this is full/overloaded or just that the mailbox is temporarily full with unread mail requiring approval (common on moderated groups where the moderator is not there).
Try contacting the group owner or moderator to see if they are aware of any problem also try Yahoo themselves.