Deadlock resource when try to drop datafile
This error is produced for various reasons stated below:


  Too high activity - Re-running the job during a less busy period can
  fix this ORA-00060 deadlock error. Poor application design - As noted
  in the trace file:  "The following deadlock is not an ORACLE error. It
  is a deadlock due to user error in the design of an application or
  from issuing incorrect ad-hoc SQL." 
  
  ASSM - This error can happen when the target tablespace is using
  "segment space management auto".  See these notes on deadlocks with
  ASSM.  Beware, ASSM can cause huge deadlocks, because ASSM does not
  perform well under heavy DML loads.
  
  RAM shortage - In rare cases this ORA-00060 deadlock error can happen
  with a RAM shortage at the Java layer or Oracle layer


One way is to use the trace file to see what is causing the deadlock.

The most common reason for which such deadlock situation occurs is that some resource(s) might be using the datafile that you want to drop.

The best possible solution is to close/terminate all the sessions using the resources on the mentioned datafile and then delete the datafile.

Another solution is by shutting down the database and then making the necessary changes.

There are many ways/queries to find the SID(Session ID) which is causing the lock. Please refer the website for some examples -->http://www.oraclerecipes.com/monitoring/find-blocking-sessions/

Analogy in Windows: It is similar to renaming/deleting a file when in use.
Hope this solves your problem !!