The cougars were not full grown, but young of the year.
Now in this case I do not believe the beasts had any real intention of attacking the men.

They were young animals, bold, stupid, and very hungry.

The smell of the raw meat excited them beyond control, and they probably could not make out clearly what the men were, as they walked bent under their burdens, with the deer skins on their backs.

Evidently the cougars were only trying to get at the venison.
In 1886 a cougar killed an Indian near Flathead Lake.

Two Indians were hunting together on horseback when they came on the cougar.