They’re probably making sure no other animals are a threat.
I like all answers so far. I’m going to delve a little further into @kritiper ‘s response about eyes. Lots of predators have “3rd eyelids.” It is a protective membrane that covers the animals eyes when biting. When the animal’s face, and eyes are in most danger. The 3rd eyelids of most animals are not really transparent. The thick membrane is usually too thick to see through. So. At the moment the predator bites, for a kill, it is essentially biting blind.
This phenomenon is most easily observed in a slow motion great white, or other shark bite. The experience seems to be similar in most species of animals that have the 3rd eyelid…
So. When biting, the animal temporarily loses sight. Then , involuntarily, they regain sight. So, a quick recalibration, is required, following the new visual information.
In regards to snakes, they cannot blink. No eyelids. Such animals are extremely “head shy.” Their instinct says “protect your eyes, we can’t cover them in the slightest.” So, these animals will dart their heads quickly, perhaps appearing confused, when going in for the kill. Lots of animals will instinctively try to injur an attacker’s eyes. If you’ve ever worked with wild birds, as I used to, you know to hold most birds at arms length. An Egret, or Herron will go straight for your eyes.
In conclusion, I would say that what you’re observing, is a combination of hypersensitivity, adrenaline, and voluntary/involuntary reactions related to eye protection….