It goes to stock pricing theory - you can basically say the total value of a company (all the shares added up) is the value of all the future profits, discounted back to todays dollars. If interest rates go up, we apply a bigger discount to reddit's future profits, so the value of reddit goes down. Since reddit is a growthy tech stock what we're kind of saying is reddit's biggest profits are a long way off so we're going to discount them a lot when interest rates are high, making reddit highly sensitive to interest rates.