As an update to this I find that if I double-click on an existing archive in thunar it doesn't open. What I did notice was excessive disk activity though.
So looking at the processes running I found the main process and also other spawned processes which were being created, then disppearing with more processes created to replace them. Sort of thing!
So I could check a process, say 12230 with ps aux, then immediately do another ps aux and 12230 is gone but the same command is now being run as 12239...... ad infinitum. Only stops when I kill the primary process, say 10057.
Hope that's clear