First thought that comes to mind is your flour. Perhaps most times you are stirring/aerating it just right and other times too much or not enough. I always use a big whisk to stir my flour well before measuring out.
Only other factor I can think of is the humidity that day.
I'm afraid I'm not enough of a baker to help you other than those thoughts which probably aren't right, just what comes to mind first.
Meddle not in the affairs of dragons for you are crunchy and taste good with ketchup.