Detaching a database with drop all connections enabled was running continuously without completing. When checked what was causing the block or lock, I found the Kill/Rollback was waiting with waittype “preemptive _os_pipeops”.
With some googling, I came to know that it is something to do with a process running at the operating system level. So the solution was to end any process like wscript or cscript or cmd running with sql server service account or whatever you think might be the process that used the SQL Database.
In my case, it was a vbscript running as wscript process running with SQL server service account. I found that in Windows task manager.