I've never seen this particular behavior, but I have seen Oracle do a number of incredibly odd things on Linux. Oracle has told me that the client on Linux is the root of the cause in many cases. What I had done in one case where mystery sessions were being spawned was to simply write a script to query the sys views (v$sessions in my case, but v$open_cursors in yours) for the offending conditions periodically through cron and to kill off the asociated processes. It will depend on your exact configuration and session management style, but this may work for you for now as well. At least until you get through this.
in reply to DBI - cursors left open in Oracle
we eventually had to run Oracle on Sun to get the stability we needed.
We speak the way we breathe. --Fugazi