Mishna Pesochim Perek 10 mishnah 5.
Rabban Gamliel requires us to mention three things at the seder and gives the reasons.
פסח, על שום שפסח המקום על בתי אבותינו במצרים. מצה, על שום שנגאלו אבותינו במצרים. מרור, על שום שמררו המצרים את חיי אבותינו במצרים.
For the korban pesach, the reason to mention it could be that the Torah says “The Lord will pass to smite the Egyptians, and He will see the blood {of the korban pesach} on the lintel and on the two doorposts, and the Lord will pass over the entrance, and He will not permit the destroyer to enter your houses to smite”
We mention matzo because we were redeemed from Egypt and the matzo was evidence for this as עיקר תוי"ט quotes the Rambam to say: עַל שׁוּם שֶׁלֹּא הִסְפִּיק בְּצֵקָם שֶׁל אֲבוֹתֵינוּ לְהַחְמִיץ עַד שֶׁנִּגְלָה עֲלֵיהֶם etc הַקָּדוֹשׁ בָּרוּךְ הוּא וּגְאָלָם,שֶׁנֶּאֱמַר וַיֹּאפוּ אֶת הַבָּצֵק because there was no time for the dough of our forefathers to rise until HKB”H was revealed to them and redeemed them as it says and they baked the dough etc.
So for these first two, there is a direct link between the thing to mention (Pesach and matzo) and the thing we ate, whereas for the moror the reason to mention seems just to be an association of words.
How do we explain the reference to the reason to mention moror from Rabban Gamliel? Is it just a word association?