Is hyperextension actually caused just by stretching or being too flexible or by a wrong movement. And why Don't ballerinas have it although they do a lot of stretching and they are too flexible.and how can stretches be Done safely

Here is a good read to explain it.

What Is Hyperextension Injury? (http://www.healthguidance.org/entry/14654/1/What-Is-Hyperextension-Injury.html)