oracle "shutdown immediate" not getting completed due to active application sessions.

I faced a strange issue when i fired the command

 shut immediate 

on one of our running oracle database instances. When i fired this shutdown command, it took longer time and the database is not closed. instead the command rolled backed by oracle itself with below comments.

alert log
========== 
Active process 31928 user 'oracle' program 'oracle@sitdb2', waiting for 'single-task message'

Active process 20462 user 'oracle' program 'oracle@sitdb2', waiting for 'single-task message'

Active process 14219 user 'oracle' program 'oracle@sitdb2', waiting for 'single-task message'

SHUTDOWN: Active sessions prevent database close operation
          Shutdown did not complete in 60 minutes
Instance shutdown cancelled
2024-12-20T12:41:57.103353+07:00

shutdown command what i executed is..

even though i have not killed the session, oracle automatically killed the session after 60 minutes of wait time (as updated in alert log).

SYS@XLTABP1:SQL> shut immediate;

ORA-01013: user requested cancel of current operation
SYS@XLTABP1:SQL> 

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