too much

Twenty minutes in to my latest loaf of Pain de Mie, and instead of the wonderful smell of bread baking, I was alarmed by the smell of something burning instead.  Peeking in, I found that the dough had forced its way out and some had fallen to the bottom of the oven.  I got the lid off and hopefully what stayed put will be as good as ever, but I need to know if the cause was:

A: Too much dough.  I added about a cup of extra flour, plus 1/4 cup mixed grains to my recipe, the first because my dough was very wet, and the other for more texture.