You are an EMT that is working at a parade on a hot day. The…

You are an EMT that is working at a parade on a hot day. There is a college marching band getting ready for their performance. The director gets everyone lined up and begins a one hour warm up where everyone is required to stand in place during the warm up. Toward the end of the warm up, you receive a call that a few members of the band have fainted from standing at attention for the long practice. The members all play different instruments and seem to have normal temperatures, normal blood pressures and only complain of being light headed before the fainting but otherwise feel fine now. What is the simplest and most likely explanation for the fainting?