Triggers don't activate when dropping table

I have one table named "object" and i many tables that are related to the table "object". When a record in "object" is deleted, the relative records in other tables should also be erased.  To implement the above goal,  i define many triggers on table "object" to make operations automatically.

 

But i find that the triggers don't activate when i dropping the table "object". So there is many garbage information left in the system which makes the system unstable.

 

Could anyone help?

Thank you!

                                                                     simon