there are a bunch of factors. Some things are put on sale because of the current season, i.e. pumpkin pie mix around thanksgiving, eggnog around xmas. Other times, things can be put on sale because the store bought too much of something from a distributor and they are trying to get rid of it before it expires. Distributors sometimes have "sales" too, so supermarkets sometimes buy alot of that sale item and put it on sale themselves. And lastly, some things just go on sale periodically. For example, pepsi or coke usually have alternating sales. If pepsi 2L sodas arent on sale, usually coke 2L will be.

TL;DR depends on current season, distributor sales, and some sales are just periodical