What causes a trigger to become disabled?

Hello,
How is it possible for a trigger to appear as disabled (in user_triggers) without explicity issuing ALTER TRIGGER ..DISABLE?

We exported the db to a client site, checked for invalid objects (there were none) and now a week later mysteriously proceses have stopped working which we have found is related to triggers being disabled.

Here's what I checked.
>Ran a simple testcase with a trigger that inserts values into specific columns of a table. Dropped one of the columns that the trigger inserts into. The trigger shows as invalid but is still enabled
>Droped a table that the trigger inserts into. The trigger shows as invalid but is still enabled.

I also thought that perhaps the trigger may not have got enabled on import however these processes have been working fine since the import
until just the past day. The import occured over a week ago.

We are 90140 on win 2k

tks for any suggestion