Mysqldump cause Sql_thread to stop.

During the backup from the library, you receive the slave Sql_thread stop alarm and check the status as follows:

(1) Show slave status output

relay_master_log_file:mysql-bin.009188

Slave_io_running:yes
Slave_sql_running:no

(2) Error log output

2017-12-13 10:35:14 61080 [Note] Error reading relay log event:slave SQL thread was killed
2017-12-13 10:44:02 61080 [Note] Slave SQL thread initialized, starting replication in log ' mysql-bin.009188 ' at position 85266734, relay log './relay-bin.017692 ' position:85266897

Check backup script for mysqldump manual

Script contains--dump-slave=2 (backup with commented change master)

There is a sentence in the parameter explanation

This option causes Mysqldump to stop the slave SQL thread before the dump and restart it again after.

Conclusion: Stop slave before backup, end of backup and start slave;

Mysqldump cause Sql_thread to stop.