Replication data resynchronisation

I will like to find out if anyone knows why this happens and if it is normal. After successfully replicating 72000000 blocks of data from primary to secondary site, replication stopped and it gave error connecting to the secondary site. Finally i successfully connected to secondary site and discovered i had 75000000 blocks of data pending, and status is showing resynchronising data.

Has anyone faced this issue? Could anyone advice on this issue?