Temps warm in alaska, thus over time the blood (and thereby marrow) of it's various indigineous creatures gets thinner (runny). This means their hearts do not need to pump as hard to circulate throughout the body. This causes the metabolism to slow, thus fat increases. Now it gets harder for them to run from point A to point B. And the males are nolonger fast enough to catch a cow for rutting, thus the population naturally decreases, and offspring inherit the new health problems. But I digress.
Now, since running is more difficult, they're more likely to eat whatever food catches their fancy as they stroll through the forests. Eventually, they happen upon a block of rotten cheese (set out by hunters trying to avenge a terrible biting of the younger sister of one of the hunters). They eat the cheese, but because the air is less crisp from cold, Marty Moose doesn't hear the snap of a branch as brother hunter steps forth to blow his brains out. The Temperature variance, however, causes the drop of the bullet to be overestimated, and it instead hits the cheese block, frightening said Marty into a cheese-induces myocardial infarction and death occurs.
Hunters argue a bit until beer is gone, and they take Marty home on the front of their hoods (the ornament phase of forgotten youth). They bleed him (more easily done) prepare him and cook him up. But without the gravy and such normally present, their own hearts begin to weaken, and eventually they are found beaten to death by a psychotic PETAphile.
This happens to a variety of other leaf-eating animals, great and small. Thus the forests begin to fill in thicker and thicker, then one summer a lightning strike burns alaska down and we have no choice but to annex Canada for future moose hunts, and as a disposal ground for bad cheese.
Simple, yes?