Mac OS X runaway process: AuthBrokerAgent
Problem solved. The AuthBrokerAgent is responsible for handling proxy credentials. If the credentials of a proxy setup are improperly stored (for example in one's keychain) then the AuthBrokerAgent can runaway.

More details and context on a follow-up posted on Apple Community forum, but the solution was to clear out bad user/passwords from the keychain and use Safari to re-ented the correct user/pass for the web proxy and check the box to have Safari save the password in the keychain.