Are you getting any error in alert file like (unable to extend temp segment)
if yes, then there could be need to create new temporary tablespace,

The data in the database use about 8.6GB, and I am wondering what could cause such growth.

it is good that temporary tablespace is showing full, no need to worry unless it is not showing any error in alert file. because the system will allocate an extent in TEMP and then keep it and manage it itself. This is normal and to be expected and is not an indication that you do not have any temporary space.

V$SORT_USAGE will tell you who's using what.