This is the best way I've seen it put without it confusing me.
The same processes that reduce ammonia to nitrite to nitrate also produce an abundance of hydrogen ions, which, if left to their own accord, acidify the water. In water from some sources that contain few "buffers" (ions that help stabilize pH by combining with excess hydrogen or hydroxyl ions), pH will tend to decline steadily just as the nitrate increases, and again regular pH testing may help alert the aquarist to impending trouble. However, in more heavily buffered water, an interesting but more threatening phenomenon occurs. As hydrogen ions are produced, they are immediately tied up by the buffer ion, and the pH remains roughly the same - until all the buffer ions are used up. At this point, the pH drops rapidly, and this sudden "pH crash" can be very damaging to fish.