If he attempted to hit you again instantly, you'd get 9 parried attacks in your log all in the same second, followed by the hit. Instead, he just uses it again when it comes off cooldown one second later, and you're still the highest hp in range (because you parried the last one and haven't taken damage). So if it's taking him 10+ seconds to land a swing, then it's not an instant re-attempt, it's just using it on the same target after the cooldown fades. Based on Questioner's screenshot, we can see that Hateful Strike really does only have a 1-1.5 second cooldown (hit at 1:43:48, hit at 1:43:49), so he's not doing it any faster when tanks parry; he's just very likely to hit the same guy on the next cooldown because they're still on top of health.
Patchwerk would target the offtanks the entire fight, the only reason we knew I was MT'ing was the endless stream of 2k hits in my log. This is the exact same mechanic as far as I can tell.
-Splug
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(Usually) Prot warrior:
http://www.wowarmory.com/character-sheet.xml?r=Illidan&n=Splug