Hello Nitesh,
As you know ASA failover is based on the exchange of hello packets on the interfaces being monitored so both ASA's can detect if one of the devices is down.
Now by default after an interface is down the mate will stop reciving hello packets from that peer and it will trigger failover.
In your case that is what happen, the status on one of the interfaces being monitored went down and failover was triggered.
Regards,
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Julio
Julio Carvajal
Senior Network Security and Core Specialist
CCIE #42930, 2xCCNP, JNCIP-SEC