For example, this happens when you start a import/export wizard to copy data to a table. it starts two process, one process selects the data and other one inserts the data. This is parallel query/process.
When you stop and exit your import/export wizard for some reason, the insert(bulk insert) goes away, but the parallel query select does not go away, which you can see using sp_who2 command.
This process will take its own time to go away, but if you kill this select process deliberately, then the EXECSYNC happens.