It is customary for pious people to fast on the day before Passover even if they are not firstborn sons. The Talmud records:

R. Sheshet would fast on the day before Passover because he was a delicate person, eating in the morning would ruin his appetite for the evening meal.

Since it is a mitzvah to eat matzah at the Seder with a hearty appetite, he would abstain from food all day.

Pious people who are in the habit of fasting frequently follow the practice of R. Sheshet and abstain from eating all day so that they can partake of the matzah in the evening with a hearty appetite.

Rav Yaakov Emden mentions another reason for this fast of the pious: it was on this day that Haman forwarded the decree to annihilate the Jews.