Quote from Bribous
...problem proc'ing Stun with Hammer of the Ancients (Thunderstrike), while using the Sever ?
Yep. I suspect the reason it doesn't work is because its actually Sever (its unique affix to be precise), that kills the mob, not HotA. The stun works only if the mob is killed by HotA.
edit: and yes, i have tested it myself :), got 0 stuns