"even senior...."
what do you expect? if it was new people leaving it would be surprising, but for someone who's been there a long time, I can think of several legit reasons for him to go see elsewhere.
he's got a good line on his resume, "senior software engineers at blizzard"... it's easier to do for him than for a newbie. so I don't get the "even senior" thing, sounds like you think the longer you've been there, the less likely you are to leave. doesn't make sense to me.