It is posture that opens and flattens the diaphragm. This has to do with controlling air. During practice we are made to sit on the edge of our seats with shoulders back for the same reason. I take it you have never been in a choir.......
No, but the first time I noticed it, the choir director was concerned too. He called it "the lunge". It was a bit of a distraction from the performance because every time he raised his hands, the girls would all lean forward.
Thriftymaid 