After a failover, the member that changed its state to ACTIVE initiates a rebuild of the tables for Dynamic Port Allocation. This process is heavy, especially if the concurrent number of NATed connections is large. (These tables are not synced between members, so they need to be rebuilt upon failover.)
During this process, Port Allocation can not be done, so every new connection that needs to be NATed will be dropped.
Dropped connections will not stay in the connection table without NAT and cause re-transmissions.