“U’shmartem es hamatzos” (Shemos 12:17) teaches that there is a requirement for matzah to be guarded/watched (Pesachim 38b). Rashi (ibid.) explains that this watching (shmirah) has two parts. First, one must watch the matzah to ensure that it does not become chometz. Second, the shmirah itself needs to be for the sake of matzah shel mitzvah. I would like to elaborate on these two elements.
Regarding the first element, although there is a general principle to guard oneself from sinning, such as guarding one’s tongue from speaking lashon harah, there is a specific halacha that matzah must be watched so that it does not become chometz. Based on this halacha, Rabbi Eliezer forbids eating matzah made by a Kusi (those who converted in a questionable manner in the times of the King of Ashur.) Kusim are not aware of the halachah that matzah must be guarded from becoming chometz. Thus, their matzah are not suitable for Pesach (Chulin 4a).