Why does my back leg bend when i do the splits?

Answer 1

Because you're not doing them properly.

The best way to get into a full split is to first straighten both legs, then concentrate on lowering yourself and stretching. A lot of dancers worry about their height and then work on straightening their legs, which makes it harder to progress and do the splits properly.

Answer 2

because you haven't stretched it out... a back bend rarely comes naturally to gymnasts . not being stereo typical but Chinese gymnasts may seem extremely bendy how ever stretch to unbelievable measures keep stretching and you will get there.