If the jeans aren't bought at all, or as much as the other brands, then the store owners would have no choice but to lower prices. It's either that option or they would return the merchandise (if possible) or simply throw away the jeans. Setting the right price is more or less trial and error. There are mathematical ways to make an educated guess, but to be fair, there are a lot of factors in play that make trial and error more effective.
For example, if the jeans start at $100 and no one buys them for a week, then the owner may reduce the cost to $75 and see if the 25% discount is effective or not. If people buy, then the price may stay there. If not, then the price may come down even further. The process is continued indefinitely. On the flip side, if demand skyrockets, then prices would likely go up.