Comment 16 for bug 1818682

Further debugging finally puts the cause of the whole issue on the table:

This issue is caused by associating or disassociating floating IPs. This operation causes the keepalived.conf of the L3 HA router to be extended on the VIP exclude list with the dis-/associated floating IP. keepalived then receives a SIGHUP to reload its configuration. Due to the SIGHUP keepalived immediately drops the monitor VIP from the HA interface of the actual active L3 router.
neutron-keepalived-state-change notices the dropped VIP and immediately takes down everything active for the L3 router including the haproxy for the metadata agent.