well that is a good question ... SMON [should] clean the TEMP tablespace whenever needed, but sometimes it doesn't ...
note that the behaviour of the temporary tablespace has changed in Oracle 8 : Oracle now creates a segment, which keeps on growing until it totally fills the tablespace, and then needed space should be reused, nevertheless I have been working on a lot of HP boxes, and some of these had the problem you have encoutered ... what I suggested is just a workaround