Why Database Trigger Can Became DISABLED

We have an application, using ddl database trigger on oracle database.Trigger depends on package and several tables, package depends on user defined function.It works on different customer sites well, but for one customer (11g?), sometimes (cannot determine when and why), trigger became disabled.

I know, that trigger can became invalid, when its dependencies changed (i think in this situation oralce probably try to re-compile it). I understand that it can changed to be invalidwhen dependencies are not valid.I know I can alter trigger to became disabled manually.

But when it can became disabledwithout explicit alter statement be run!? Are there some conditions / circumstance when Oracle changed the trigger to be disabled?We'll ask the customer DBA team to enable audit on trigger, but it will take time to be done..