What caused my bread to sink after rising

I'm using my breadmaker to make a multi grain bread.  It was looking great, having risen almost to the top of the bread pan.  Then, it started to sag.  At first, the middle collapsed a little, then, the sides almost caught up.  The situation is not terrible as the bread will be finished shortly and it still rose enough, however, it was around an inch taller.

What caused the drop?  Itmust be too much, or too little of some ingredient.

Thanks