I’ve heard, cannot confirm, that this inyan is to commemorate the ness that happened to the Maharal. According to this version one of his shamos’s keys (to the Aron Kodesh) kept dropping off the shelf. The Maharal instructed him to visit the shul and check to see if the Aron Kodesh was all in order. Upon inspection he noticed a bottle of blood, replaced it with wine, thereby saving his entire kehilla. Again that is this version.
So in order to commemorate the ness they form a key on the challah on the Shabbos immediately following pesach, because one is not allowed to make a form on a matzah.
Again – cannot confim.