If you've ever seen a nature documentary - or seen bears in real life - you might know how this situation would normally work. The mother would take the babies on her back, and complete the swim.
This wasn't a normal situation, though. The mother went against what most of us would call maternal instincts, threw the cubs off her back, and attempted the crossing alone. It looked like an act of extreme selfishness; sacrificing her cubs to save her own life. It made for very shocking viewing.