Market fluctuation meaning prices go up and down with time.
It's mainly because they see a certain price when in the inventory, let's say 1.70€, and they want to sell it in the instance, and they put 1.69€.. and someone else does the same thing, but he sees 1.69€ so he puts 1.68€ and so on..
people are buying counter strike GO in bulks of 100+ per person, so obviously the seller has to sell the keys to get more funds to buy counter strike to sell. Same thing happens every major steam sales (e.g. summer sales). In this case, Counter strike is the 2nd most popular game after dota 2 (and the number 1 paid game on steam), so yeah the price will drop at least until the sale is over - in a few hours. Watch as the price will go up back to $2.50 again until the next sales.
It always happens when CS:GO is on sale. CS:GO keys are more stable thats why I prefer them.