The reason behind this is that sesame seeds are one of the foods that can cause a person to regurgitate and causes one to feel full (of eating the meal). The Alter Rebbe in his Shulchan Aruch HaRav (608:8) writes:
On the day preceding Yom Kippur, even during the morning meal, one should eat only foods that are readily digestible (so that he will not be [overly] satiated),39 e.g., fowl and fish, so that he will not be satisfied and proud while praying on the day preceding Yom Kippur.
Sesame should be avoided, since it can cause reflux. It can also cause nausea, vomiting, coughing etc... See also here:
Sesame is the ninth most common food allergy among children and adults in the U.S. The edible seeds of the sesame plant are a common ingredient in cuisines around the world, from baked goods to sushi. Several reports suggest this allergy has increased significantly worldwide over the past two decades.1
When a person with an allergy to sesame is exposed to sesame, proteins
in the sesame bind to specific IgE antibodies made by the person’s
immune system. This triggers the person’s immune defenses, leading to
reaction symptoms that can be mild or very severe.
Starting January 1, 2023, sesame will become the ninth major allergen
that must be labeled in plain language on packaged foods in the U.S.
While some manufacturers may begin labeling for sesame sooner, they
are not required to do so.
Thus, sesame should be avoided, since it can cause one to regurgitate.
Shulchan Aruch HaRav 608:10
One should not eat sesame seeds on the day preceding Yom Kippur, [lest] they cause regurgitation on Yom Kippur.
Also, garlic and eggs should also be avoided, since these are products that causes the human body to produce sperm (see: footnote 9).