Shelah HaKodesh: Yachatz
Why is the middle Matzah broken rather than the top or bottom one? There are two “Berachot” that are recited before
eating Matzah at the Seder: “Hamotzi” – the blessing for “bread” – and “Al Achilat Matzah” – the special blessing over the Mitzvah of eating Matzah on the first night of Pesach. The Halacha is that whenever we recite Hamotzi it is preferable to have a whole loaf of bread or Matzah before us, whereas the Beracha on the Mitzvah of eating Matzah is preferably recited over a broken Matzah, symbolizing that it is “Lechem Oni” – “The bread of poverty (or affliction).”
There is a Halachic principle which states, “One may not pass over a Mitzvah (or a Mitzvah-object).” Generally speaking, all Mitzvot should be done in the order in which they present themselves to us (and all Mitzvah-objects should be used in the order in which they are brought before us). This rule obligates us
to recite “Hamotzi” over the first Matzah that we see when we are ready to eat, i.e. the top one in the pile of three, and because of the Halacha cited above this Matzah must be whole, not broken. The blessing of “Al Achilat Matzah” must be recited on the second Matzah we see, i.e. the middle one, and this one should be broken.
If we had broken the bottom Matzah instead of the middle one, we would have to pass over the middle (whole) Matzah to reach the broken one, and this should not be done.
(Hagadah Shel Pesach MiBa’al Shlah HaKadosh)