Most GMK keycaps (and a lot of PBT keycaps) are done through group buys. Generally a group buy is open for about a month and collects orders and payments, and then the group buy runner submits one big order to GMK. These days, due to high demand and a long production backlog it takes about 6-9 months for keycaps to arrive after the group buy order is placed. Usually vendors will place orders for a limited number of extra sets, and resell those at a higher price once the group buy is over, and I think that's what you're referring to with Drop. GMK sets are usually $110-$130 for a base set when done through a group buy, and ~$180 when buying extras from a vendor after the group buy period.
Group buys are run because setup costs are high, and there isn't a ton of capital for vendors to buy large stocks of keycaps at once. With injection molding a lot of the time and cost is in set up (mixing the plastic and dye, getting the color correct, setting the machine up correctly, etc.), which is why there's minimum order quantities (generally 150-250 for GMK, depending on if custom colors are used), and which is why orders are done in batches. Popular sets re-run when the demand for them is high (Usually pretty apparent when used prices for sets hit well above their original price).