It's obvious isn't it?
There's tens of millions of players, and only a limited number of these party hats. People obviously buy them and there aren't any more coming into the game, giving them a rare touch. If you knew anything about the market, you'd know that the reason they went to shit is because when Jagex removed the pking/staking, the richest players in Runescape dumped their phats into GE until eventually they dropped (In desperation to sell, they placed a min, causing party hats to drop). This worried the other partyhat holders so everyone threw in their party hats to sell, none of them did. This became a vicious circle and soon party hats wouldn't sell at all.
Now that Jagex forced price stabilization and bringing purples down like %40 of their price, people take interest. The price will only go so low for any item before people stock up, especially on something with such a low population such as a Party Hat. Once everyone realized people are buying them, they all wanted one, causing a reverse in how they crashed, and shooting price back up.
So before you come in here like you're a fucking hero because you spam a lot, think about what you're typing before looking like a fool in someone elses thread you have no business in. Maybe do a little research if you plan to bring someone down and "LMAO" at someone who may appear to me new, you bloody prick.