This article explains why members of a group may be labeled as "bouncing".

If messages sent to a member are consistently returned to Yahoo! Groups with an error, that member is considered "bouncing." Their Yahoo! Groups account is deactivated, and group messages won’t be delivered to the account.

Once an account is considered bouncing, Yahoo! Groups will send a series of test messages to the member. If the test messages are returned as undeliverable, then the account will continue to bounce. However, if a message is delivered and not returned as undeliverable within five days after it was sent, the account will automatically be reactivated. Members may also manually reactivate their accounts by visiting their My Groups page.

Note: If Yahoo! Groups determines that an email address has been bouncing for more than 6 months, we may remove it from our system.