A mother bear will do anything at all to protect her cubs when possible. Bears are great swimmers, but when one brown bear tried to cross a lake with her cubs, she realized she had overestimated their ability to swim. Rather than stay with her struggling babies, she had to leave them behind to save her strength.
Usually that would be the end of the story. It's a fact of life that nature is sometimes cruel. Thankfully, some people came along at just the right moment.