Well, there is this also- From the time rut starts, until April or May, most native hunters will only shoot cows. The bulls are skinny- they want fat cows. Which of course are all pregnant, if they haven't been run too much via snow machine to the point they abort. Even if they don't, the calves that are born are likely to have been impacted, leading to lower survival rates.
But don't discount weather and habitat change, forage degradation or heavy browsing.
