MySQL relay log rotates too soon and too often

I have a simple master/slave setup where the slave is just a backup. Sometimes I notice that the relay log on the slave rotates too often and too soon, when it is still very small:

4:00 PM
              Master_Log_File: mysql-bin.000051
          Read_Master_Log_Pos: 860318633
               Relay_Log_File: vps-relay-bin.000350
                Relay_Log_Pos: 922

5:00 PM
              Master_Log_File: mysql-bin.000051
          Read_Master_Log_Pos: 860324526
               Relay_Log_File: vps-relay-bin.000351
                Relay_Log_Pos: 6176

7:00 PM
              Master_Log_File: mysql-bin.000051
          Read_Master_Log_Pos: 860331661
               Relay_Log_File: vps-relay-bin.000352
                Relay_Log_Pos: 7418

9:00 PM
              Master_Log_File: mysql-bin.000051
          Read_Master_Log_Pos: 860333988
               Relay_Log_File: vps-relay-bin.000353
                Relay_Log_Pos: 501

Notice that the master log is the same and its position didn't change much. But the relay log rotated several times, and didn't get very big. MySQL .err shows:

2014-03-23 14:35:14 4971 [Note] Slave I/O thread: connected to master '<removed>',replication started in log 'mysql-bin.000051' at position 860316289
2014-03-23 15:59:50 4971 [Warning] Storing MySQL user name or password information in the master info repository is not secure and is therefore not recommended. Please consider using the USER and PASSWORD connection options for START SLAVE; see the 'START SLAVE Syntax' in the MySQL Manual for more information.
2014-03-23 16:59:52 4971 [Warning] Storing MySQL user name or password information in the master info repository is not secure and is therefore not recommended. Please consider using the USER and PASSWORD connection options for START SLAVE; see the 'START SLAVE Syntax' in the MySQL Manual for more information.
2014-03-23 18:59:56 4971 [Warning] Storing MySQL user name or password information in the master info repository is not secure and is therefore not recommended. Please consider using the USER and PASSWORD connection options for START SLAVE; see the 'START SLAVE Syntax' in the MySQL Manual for more information.
2014-03-23 20:35:53 4971 [Warning] Storing MySQL user name or password information in the master info repository is not secure and is therefore not recommended. Please consider using the USER and PASSWORD connection options for START SLAVE; see the 'START SLAVE Syntax' in the MySQL Manual for more information.
2014-03-23 21:35:55 4971 [Warning] Storing MySQL user name or password information in the master info repository is not secure and is therefore not recommended. Please consider using the USER and PASSWORD connection options for START SLAVE; see the 'START SLAVE Syntax' in the MySQL Manual for more information.
2014-03-23 22:35:57 4971 [Warning] Storing MySQL user name or password information in the master info repository is not secure and is therefore not recommended. Please consider using the USER and PASSWORD connection options for START SLAVE; see the 'START SLAVE Syntax' in the MySQL Manual for more information.

Master's log doesn't show anything.

What could be happening here?