It isn’t the toolbar that is actually being flagged/blocked but the zynga.com domain being blocked by the network shield.
This normally happens if the domain has been hacked or has become a distribution point for malware (usually as a result of hacking the site). If there are a high number of alerts with the web shield, then that information would be collated and result in the domain being added to the malicious sites list for the network shield.
I don’t know why the domain has been added to the network shield list as I’m only an avast user like yourself.