Why do lungs collapse after a person is crucified?

I recently heard a report where scholars were quoted as saying that most people who were crucified often died from collapsed lungs. Considering that a crucified person is held up on a cross by iron spikes through the feet and wrists (not palms); how is it that the persons lungs would collapse? Also, just how exactly would a scholar on this matter be able to make an assessment like this?

Thank You

Source: http://www.holytrinity.ok.goarch.org/Interesting%20Stuff/Special%20Communication%20Plus%20Picture.html

IIRC there is a spot in the palms that could support the weight of crucificition, though recently discovered (re-discovered) though the wrist seems more obvious.