Meaning of Leaning in the Seder Ritual
AISH RABBI: Rather than a tradition, reclining while
eating the matzah and drinking the four cups of wine is a halachah. Leaning
symbolizes freedom and aristocracy. It is first recorded in the Mishnah (c. 200
CE), but it dates back much farther than that.
Only the Jews in Egypt were commanded to eat in a state of readiness to leave; they were indeed getting ready to leave Egypt. But that command was specific for those people and for that year alone.
Sources: Talmud Bavli Pesachim 99b, 108a