The Source Set deletion is part of running a reference pattern where the set of sources for the reference is deleted.

More on running reference patterns can be seen here:  8.0.x:  CMS - Dependencies - Rules tab 

The loglines generated by this task is as follows

CAST-MS log file

Section of CAST-MS log file dealing with Create Source Set step as shown in the log above.  

Starting Delete Source Set  

INF: 2016-09-13 16:35:09:              starting Task Delete  source set for My Reference Pattern 

 INF: 2016-09-13 16:35:09:              Task message: No Task message

INF: 2016-09-13 16:35:09:              No associated log file

INF: 2016-09-13 16:35:09:              ending Task Delete  source set for My Reference Pattern