F5 HA status change reason

Look for sod entries in the ltm log. When the unit goes to active or standby it will log this fact. You may need to look at the entries immediately preceeding the active/standby message. Also look on the peer unit for the same thing, as sometimes the peer will hold the key to a failover. You also may wish to look at the ha table to see if something has moved to fail.

Failing all that, take a qkview and open a support case.