A new species of bear develops the temperament and train-ability of a domesticated dog.
People love the domesticated bears. They're so big and cuddly. The bears quickly become the most desired pets in the world.
Bear breeding farms sprout up all around the world where the bears are subjected to awful conditions. There's a general outcry against this but people keep buying the bears and the cruelty doesn't stop.
Many of the bears are abandoned by their owners when they become adults and the owner realizes how expensive feeding a fully-grown bear is. And that's before we get into the issue of the piles of poop the animals leave behind!
Although feral bears are smaller in size than their wild cousins, their sheer numbers makes their dominance in the wilderness inevitable. Habitats are destroyed and wild bears all become further endangered as the feral bears encroach on their territory and compete for food.
Animal control in every part of the world needs its budget doubled or tripled to deal with the problem, but many governments are slow to act on the issue.
Feral bears without fear of humans become common sights in small towns and the outskirts of cities, where they reap unmitigated havoc and destruction.
The governments of the world finally respond as the feral bear populations make their way into wealthier areas, but their responses tend to punish responsible bear owners as much or more than they actually address the bear-related problems.
In America, the Supreme Court rules that the right to "bear arms" includes the right to own a bear, so the few places that manage to have sensible restrictions on ownership are forced to drop them.