I ran nodetool decommission on a node from a 4 nodes cluster as follows:
nodetool -h decommission 

But it never completes & doesn't show any messages, on seeing the logs I noticed:
java.util.concurrent.RejectedExecutionException: ThreadPoolExecutor has shut down
at org.apache.cassandra.concurrent.DebuggableThreadPoolExecutor$1.rejectedExecution(DebuggableThreadPoolExecutor.java:60)
at java.util.concurrent.ThreadPoolExecutor.reject(ThreadPoolExecutor.java:767)
at java.util.concurrent.ThreadPoolExecutor.execute(ThreadPoolExecuto r.java:658)

on viewing netstats on this node I can see the node in LEAVING mode 
Nothing streaming to /
Nothing streaming to /

Not receiving any streams.