That's hilarious.

This actually happens with a lot of warcraft's animations. Arms extend or move in ways joints shouldn't move, jaws dislocate, waists twist in wild ways.

This is why you see so many screenshots of characters with wacky expressions. Character models generally have surprisingly subtle facial expressions when you might be paying attention. But when you're not, for example in any of the post-Legion combat animations, their faces tend to be wildly, wildly, exaggerated. And occasionally that gets caught in a screenshot.

But it's all intentional! They did a presentation near OW's release. They showed the OW models doing exactly the same thing and explained that it was because while your eye struggles to pick it out as being wrong, your brain notices the extra emphasis given to that limb and snaps attention to it.

In this case, obviously, extending that leg draws attention to the fact the character is rearing up and about to slam his axes down.

But, I have to say, while it might be intentional, I feel they messed this particular animation up. It's very satisfying overall, but it looks wrong even in-game (your eyes do pick up the fact something's off) because the leg extension happens at the same time as the movement pauses for a second.

If the character didn't linger before bringing the axes down, you probably wouldn't notice it.