Its a while since I have used ASAs in transparent mode so not sure if there will be any nuances or gotchas with this setup. However, saying that, if you have two ASAs for HA, do you also have 2 routers? If so, are they running HSRP/VRRP/GLBP? If the setup is something like the below:
switch ----- ASA1 ------ NSP1 -------- Router1
switch ----- ASA2 ------ NSP2 -------- Router2
and the link between NSP1 and router 1 goes down (using HSRP as example), Router 2 will no longer get hello messages from Router 1 and will assume the active state. The switching will take care of the rest, and traffic will start to flow through ASA2.
1) Could you provide further information as to how you have the active/standby configuration of the ASA devices setup (I thought that in transparent mode they were essentially active active and traffic could flow through both, but as said, its been a while!)
2) Could you confirm further details about the surrounding environment (such as any specific high level STP, HSRP etc details)