HA gone standby/standby

Check if you have vlan-failsafe configured on vlans and if the action is set to failover. If so, the vlan-failsafe action will try to trigger a failover when the bigip sees no traffic on the vlan. Check the logs to see for messages like

Sep 23 04:54:52 dc0-lab-ltm0a.test.com notice sod[5532]: 01140029:5: HA vlan_fs /Common/VLAN27 fails action is failover.
Sep 23 04:54:52 dc0-lab-ltm0a.test.com notice sod[5644]: 01140029:5: HA vlan_fs /Common/VLAN27 fails action is failover.

You will have to remove vlan-failsafe before making any changes to trunks, vlans or selfips, which affects the traffic flow.