Sharks can't swim backwards. It's not a matter of intelligence, but rather physical impossibility. Their fins are not flexible like other fish. They can go backwards by stopping and falling backwards, but the sharks in this movie clearly don't do that.
It's remotely possible the sharks were genetically modified to swim backwards, but that doesn't explain why the scientists are so surprised that the sharks can go backwards, or why they don't explain it to Jackson's character when he asks about it.