I will use examples to help you understand:
For rares
To understand this you need to know that everyday more and more cash is pumped into runescape by drops, quests, etc.
Lets say there are only 20 Bandos Godswords( which I will refer to as BG) in all of runescape, and one person owns all of them. There is also only 5B gold in all of Runescape. You cannot get any more Bandos godswords, the 20 in circulation is all we have. So a lot of people are going to want one of those godswords, wether to feel important that they are one of only 20 people in the world to have one, or to actually use it for some purpose. Now everyone will try to buy them off the person, and it will basically turn into an auction, and the person with the godswords will sell, lets say 19 of the 20, to the 19 highest bidders. Now there are 20 people with this 'rare' item, and none of them want to sell t. Time will pass and sooner or later some of these BG users will quit or lose their BGs somehow. Now lets say that 10 of the BGs are lost either by the owner quitting or high alching it, and only 10 remain. Now some of the remaining BG owners will want to sell their BG for some nice profits since they realize that there are only 10 BGs left, and the money in circulation has increased now to 10B in all of Runescape. So now not only are their items 'rarer' the prices have also inflated, they can sell their items back at any item for upto triple what they bought it for, due to the huge demand, yet small supply, and no more coming into circulation of the market.
For Normal items
I will make this one simple and short.
When Bandos armor first came out it was worth over 100M, and there were only a few sets. Over time though people got more and more Bandos drops, and it wasn't sought after as much. So due to the rising supply, and lowering demand, they reach equilibrium price which is basically know as its 'stablizing price'. Now the reason they are going down again is due to two reasons. One is the new armor sets being introduced, such as Torva, now everyone wants this armor instead of Bandos, causing the demand to take a hit, all while the supply keeps increasing. The second reason is all the new gold put into circulation, mainly through combat drops from bots, which I hope is self-explanatory.
Note: Although it is widely believed by most 12 year old botters, highly botted items such as essence, vials of water, and clay, do not cause inflation unless they are being sold to a NPC store. As long as these items are sold to other players there is just more supply of that one item, it does not create, nor destroy any gp from circulation.
I hope that made sense, I am not a great at explaining, so that was the best I could do. You need some basic economic knowledge to understand what I was trying to say.