To ensure the normal forwarding of large traffic, a network administrator of a company uses...

To ensure the normal forwarding of large traffic, a network administrator of a company uses two firewalls to implement hot standby. As shown in the following figure, when the configuration is complete, it is found that when A of the two firewalls fails, the data stream being transmitted before the fault has been seriously lost, but the newly transmitted data stream can work normally after the fault. What could be the cause of this phenomenon?