The sad music that accompanies the video is telling. Also, look how (s)he is opening/closing the mouth repeatedly from 0:35 until the end. A bear is not a dog, but I found this, Causes of Dog Opening and Closing Mouth Repeatedly
Causes include:
- A sign of nausea
- A symptom of distemper
- A neurological issue
Between that and the foaming at the mouth that you noticed, I think it's very plausible to think that the underlying problem here was some sort of illness. Too bad NatGeo apparently didn't bother to get a veterinarian's opinion.
I know but it's really sad to see a wild animal suffering...I grew up in the wild and it's tough. Nature is divinely indifferent to the cycles of birth, death, and illness....unlike us humans.