We do not know what causes this. Normally, the cursors will be closed automatically when you finish using them. This has been a rare occurrence. It can be fixed by terminating the user's Oracle session then they can log in with a blank slate.
In one case, recommending that the client close (or "X" out of) application windows instead of using the green Back button to navigate reduced the incidents of the "Trigger raised unhandled exception ORA-01000" error significantly.
Alternatively, having the DBA team increase the "open_cursor" parameter to a higher value (i.e. from 1000 to 2000) may also decrease the incidents of the error.