After all this is struggle mama bear chose to save herself over her cubs. Yes, here is the heart wrenching twist. she could have chosen to take her cubs on her back and swim across the river, instead she chose to abandon them, leave them behind and save herself. Yes, it was act of selfishness and yes it is hard to believe. But as John Darwin says, the nature is only for the fittest, which means only the fittest and most clever can survive the nature.
The mama bear moved across the river leaving her cubs behind which was a grieving and devastating situation for the little cubs.