Even without her cubs, the mother was clearly finding it a struggle to battle against the current. She gave up on making it to the shore, and changed direction. In the middle of the lake was a tiny island.
If she couldn't reach the shore, she could at least make it there, and get herself out of the freezing water. The mother set off toward the island as fast as she could. Without her, and unable to swim, the cubs had been left to fend for themselves.