Why ssh session gets disconnect after restarting iptables service ?

Issue

• After taking a ssh connection to a system and then restarting the iptables service , the ssh connection/session breaks, gets disconnect. Its found that before restarting iptables service if ip6tables service is stopped or command ip6tables -F is run then then ssh connection doesn't get break or disconnect.
• Below is how it affects, after restart there is no console and session breaks:

root@example> service iptables stop
iptables: Setting chains to policy ACCEPT: filter          [  OK  ]
iptables: Flushing firewall rules:                         [  OK  ]
iptables: Unloading modules:                               [  OK  ]
root@example> 
root@example> service iptables start
iptables: Applying firewall rules: WARNING:                [  OK  ]
iptables: Loading additional modules: nf_conntrack_proto_sc[  OK  ]

Connection closed by foreign host.

Disconnected from remote host(10.255.X.Y:22) at 11:12:47.