I believe Valve runs this like Aliexpress does with their bonus system. You spend money (I did spend ~30 Eu on the market) and it's more likely to get rewarded with the random bonuses or drops in the case of CS GO.
It happening to me before when I stopped playing the game for a while. The drop after I started playing again was an valuable case. It could be a random luck, but most online stores like to reward big spenders and those with good ROI.