Most of the time when you have an in demand item they protect it in your cart for a specific amount of time (usually a couple minutes max) so you can get out your wallet, enter cc details and shipping, etc.
Anytime I've bought a "drop" they protect carts temporarily. Hyped shoes, computer parts, game consoles, etc. Part of it is to prevent bots from sniping the actual people buying.
It said 99 in stock. I hit add to cart, it forced me to make an account, and then I had an empty cart and it was sold out. I'm not mad, it's just not what I'm used to. I just need to be faster, the link was in here for almost a half hour before I clicked and I'm sure there was stock for close to an hour.