Woops, sorry. Too much cheese book reading
. So after the bacteria do their thing and bring down the pH, they run out of food or can't live properly because of salt and/or the lower temperature. Over the weeks, they start dying off. When they die off, their cells break apart. Inside those cells are enzymes (called proteases), and the enzymes start working on breaking up proteins and producing a bunch of other stuff (like smaller proteins, amino acids) that give cheeses their flavor and aroma. That entire process of breaking down proteins is called proteolysis.
So long as bacteria have a happy environment, they'll keep making acid. Bacteria do not like salt, and like a range of warm temperatures, depending on if it's meso or thermophilic. For example, for cheddar, after you salt the milled curd, the acid development doesn't stop entirely, but pretty close. It's enough to press and keep the pressed cheese at room temp without worrying that the acid will keep developing.
marianstock, to answer your question, a few ways exist for measuring pressed cheese pH. You can cut off a piece and put your pH probe directly to it. Or you can take a piece, puree it with reverse osmosis or distilled water that has a pH of 7, and then measure the pH of the puree.