why snapshot stoped?

MY oracle database is 11g.

Recently, My db's snapshot was stoped. And so I started bottom command.

exec sys.dbms_workload_repository.create_snapshot('ALL');

but after 1 week, It hapened again.

and so I execute the start command. and I saw m00XX.trc.

bottom trace file was created.

why snapshot stoped ?

Trace file /app/oracle/diag/rdbms/sky/SKY1/trace/SKY1_m000_17855.trc

Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - 64bit Production

With the Partitioning, Real Application Clusters, Automatic Storage Management, OLAP,

Data Mining and Real Application Testing options

ORACLE_HOME = /app/oracle/product/11.2.0.3/SKY

System name:    Linux

Node name:      skydb01.daelim.co.kr