It could be a hi-resistance joint causing excessive heating. That may be a bad / loose crimp if you're using that type of terminal end or an oxidized connection at the SSR screw. I'm not sure if localized heating at the terminal would shorten the life of the SSR. I would say remake the high voltage connections so everything is clean & tight and see what happens and if the heat on the heat sink side seems normal.
yep, what P-J said - he beat me to it!