Voters will become unsubscribed if they have clicked the "Unsubscribe" link at the bottom of the announcement or reminder emails. We are required to include the unsubscribe link to meet the requirements of the CAN-SPAM Act.
It is quite possible that the voter clicked this by accident, or the email was forwarded and the recipient of the forward clicked unsubscribe, but the email was coded to the original recipient. Another possibility is that the voter has a filtering firewall that is hyper-aggressive, and it had a robot that followed the link and unsubscribed them.