It's an amazing phenomenom. Something that no deer biologist has the answer to. And don't think for one minute they don't do it on purpose. I've adopted a new technique this season for situations like that. I'll get all my stuff packed. Get situated just like you would when you get down. Check the time, put the grunt away, rattling horns, etc....... lower the bow down. Get up and put your foot on the first step. Then immediately sit back down and raise the bow and knock an arrow.
You know what happens? You sit another hour and when you get down for real .........."IT" happens. Go figure.