A failover event is recorded in the log messages:

Jan 01 00:00:50 BIG-IP notice sod[5855]: 01140029:5: HA proc_running tmm fails action is go offline and down links.

Jan 01 00:00:50 BIG-IP notice sod[5855]: 010c0050:5: Sod requests links down.

Jan 01 00:00:50 BIG-IP notice sod[5855]: 010c0054:5: Offline for traffic group /Common/traffic-group-1.

Jan 01 00:00:50 BIG-IP notice sod[5855]: 010c003e:5: Offline

Jan 01 00:00:50 BIG-IP notice logger: /usr/bin/tmipsecd --tmmcount 4 ==> /usr/bin/bigstart stop racoon

Jan 01 00:00:50 BIG-IP info lacpd[5502]: 01160016:6: Failover event detected. (Switchboard failsafe disabled while offline)

Jan 01 00:00:51 BIG-IP err bcm56xxd[5296]: 012c0010:3: Failover event detected. Marking external interfaces down. bsx.c(3633)

Jan 01 00:00:51 BIG-IP info bcm56xxd[5296]: 012c0015:6: Link: 1.1 is DOWN

Jan 01 00:00:56 BIG-IP notice mcpd[5318]: 0107143c:5: Connection to CMI peer 10.0.0.3 has been removed

Jan 01 00:00:56 BIG-IP notice mcpd[5318]: 0107143a:5: CMI reconnect timer: enabled

Jan 01 00:00:56 BIG-IP notice mcpd[5318]: 01071431:5: Attempting to connect to CMI peer 10.0.0.3 port 6699

What is the cause of the failover?