I spent the last three days in Everfrost & Permafrost with a level 48-50 6-box (some mentored down) with a lvl 49 Shadowknight leading the team. I puposefully got aggro on the SK, then would pause the bot on the SK and let one of the other team members get aggro. I tried this numerous times against numerous groups of enemies in both zones. Throughout all these fights, the SK never activated his Evac, even when loosing aggro 3 Times during one of the Epic X2 fights.
I'm not sure what's going on with your setup, but I can only recommend deleting all SK related files (spell list, class file, custom_char_name file, etc) and re-download them from the SVN.
If all else fails, backup your current 'scripts' folder, and re-setup from scratch.
g'luck