It only takes one big sale to drive up the price of an item. One time it sells for above average price (for whatever reason), everyone watching/bidding suddenly thinks "i need to get one of these while i can afford it", they then drive the price of the next one and so on and so forth.
Its a bit of a self fulfilling prophecy eg Someone says that a bank is going out of business (even if it is not), everyone gets worried and takes their money out of the bank resulting in the bank going out of business
Give it a bit of time and prices will normalize after the spike.