Read my words again there Bart. I didn't say it WILL cause flat primers, I said flat primers can be caused by this. If you didn't get flat primers with excess headspace, good for you.
mrawesome22 explains it pretty well here in post #3.
http://thefiringline.com/forums/showthread.php?t=484283
You can also find reference to it in this article.
http://www.loaddata.com/articles/PDF...20May%2011.pdf
Here post #7.
http://thefiringline.com/forums/showthread.php?t=496694
Here post #4.
http://thefiringline.com/forums/showthread.php?t=454722