I tested it some more and it is every 3rd attack. It will do it with Strike, swipe, slash, follow up, focus, eviscerate, and spin.
Please note the they claws are still out, but he does that 1/4 squat, whip arms out to side animation. This is what I reference as "whip claws out"
I am going to attempt to link a 40 second video of 1 mob attack. I hope this gives you a better idea.
There is one point that I use a Bio click in the video, but even then he does not do the "whip out" animation afterwards, but I could be wrong.
https://drive.google.com/open?id=1nP6Y2qk0KZD1etLzpgi2Cqh0APcgLq7i
-Elrodvoss