Why do trade discount rates change

Trade discounts change according to payment type length of full payment and often quanity of the trade product

Supply and demand, and economies of scale.

For example, if everyone wants to buy carrots, they will be more expensive. If all the carrots in the world get carrot fly, it will get more expensive. Alternatively, if I can buy more carrots than you, this means that the cost in selling these carrots will reduce and the greengrocer will be able to discount the product.