Investigate postgres 9.6.3 repmgr lag 4.0.4

Hi,
During the weekend (on friday) one of my slaves suddenly stop syncing with
the primary. I have 1 master + 2 standbys. When I arrived this morning to
work I saw that one of the slaved has a big lag :

postgres=# SELECT CASE WHEN pg_last_xlog_receive_location() =
pg_last_xlog_replay_location() THEN 0 ELSE EXTRACT (EPOCH FROM NOW() -
pg_last_xact_replay_timestamp()) END AS log_delay;
log_delay
---------------
173781.573457
(1 row)

The only rows in the repmgr.log :

2018-06-23 19:02:48] [WARNING] unable to connect to upstream node
"ptkpl-psgsqldb1" (node ID: 5)
[2018-06-23 19:02:48] [NOTICE] node has recovered, reconnecting
[2018-06-23 19:02:48] [NOTICE] reconnected to upstream node after 0 seconds

Now, How can I further investigate it ? my wal_keep_segment is assigned to
100 but since friday 261 wals were generated so I guess I dont have another
option but to sync the node again. However, I want to understand why it
happened. What can you advice me to check ?

Moreover, I have 2 further questions :
1)Since friday 260 wals were generated, it doesnt sounds like too much ? I
didnt have any unusual work. A new wal is generated everytime the older one
is getting full or every switch right ? Is there a way to set the size of
the wal ?
2)Any recommendation regarding the wal settings ? My current settings :
max_wal_senders=10
wal_keep_segments=100
max_wal_size =4GB
min_wal_size = 1GB
checkpoint_completion_target = 0.7

Thanks