I've gotten about 3 of them from him, and 1 from Alad V
and no attacks yet.
From my experience it's not terribly hard to get Harvester (Alad V's pet thing) to attack you. You just need to spam highish level Corpus missions.
Stalker on the other hand, seems 100% random. He used to attack you more often if you had low level equipment, but I think DE removed that behavior.