There are many literary expressions that simulate or evoke the feeling of nervousness, apprehension, or anticipation. Having butterflies in one's stomach or ants in one's pants describe nervousness and ansiness well because it is easy to imagine how it would feel for those things to actually be where we say they are.
The same applies to yeast. Imagining a mass of bread dough rising in a bowl inside our chest evokes an image of pressure and angst. The tension is building more and more and the anticipation is rising!