What you're describing is natural behavior for rollback tablespaces. If you look at tablespace manager it may show you the segments that are at the end of the datafile.
Even if you do drop the offending segments, your tablespace will be back to the behavior in short order. The proactive method is to size rollback segments appropriately to minimize extensions. Check the documentation or tuning books to get the scoop. Do this primarily by setting minextents to 20, and picking an extent size such that 20 of them will accomodate your largest average transaction. Set optimal = minextents*initial.