Speaking out of an excess of ignorance, I believe the key is certain grains which have touched water. Because grains touching water - particularly in the time those rules were set down - were likely to sprout and/or ferment. But again, I am woefully ignorant on this topic, and could be entirely wrong in my historic speculations.Jews observing Passover. I'm familiar with the no leavening, not so much with avoiding certain grains.
Leavened breads were usually made with a starter, and as anyone who maintains sourdough starter at home knows, it also ferments.
The interpretation of matters of faith is well beyond my expertise.