Maybe there's a big player buying them up progressively, in greater quantities when the price is towards $4.80, and in lesser quantities (or zero) when the price moves towards $5.20.
The BitCoin economy has grown significantly, more services have been developed, there is greater liquidity with things like BitInstant, Bit-Pay, foreign currency trading/arbitrage bots have been deployed, etc. and therefore more capital has moved into bitcoins. This has resulted in it taking at least 10-20x as much capital to move the price $.10 as it did 12 months ago.
Additionally, there is tremendous potential buying still sitting on the sidelines. I have had numerous discussions with people whose net worth is in excess of $100m and even helped one make a five figure bitcoin investment. BitCoin has made a proof of concept and proven itself worthy of additional capital which is starting to flow in. I know of at least 4-5 potential six figure bitcoin investors that are seriously sniffing around and other VCs who are looking for some serious BitCoin related businesses to invest in.
I'm glad you brought up the fact that big players are coming in. That's another reason why we should have more volatility. Market cap of bitcoin is roughly $45 million right now. It wouldn't take anything for those big players to move this market. If a big player bought any significant amount of bitcoins the price would soar. Volume is roughly 30k bitcoins a day which is about $150k of bitcoins. $150k is pocket change for "people whose net worth is in excess of $100m." You can't have all of the activity described above and not have significant price variation. So far the only thing we haven't ruled out market manipulation by insider trading. Those players may realize this happening and be wary of entering the market now. Those guys are professionals and study trading patterns and fundamentals under a microscope. Why would they buy if the market is rigged? Arbitrage bots don't explain it. They are small players. Arbitrage bots take advantage of subtle price differences across different exchanges. Keeping price at a given level on one single exchange would not be profitable. With 10-20x more capital invested we should see double digit bitcoin prices at the minimum.
Last year we were well above $10 until negligent security practices brought down the entire exchange. Bitcoin was itself was tarnished by investors who concluded bitcoin was at fault. Guilt by association was their first conclusion. We had far less interest from big players then and yet price is lower now with high interest from big players. Before the exchange became a monopoly you could take your money to an exchange that wasn't rigged. For various reasons we lost market competitors. Price stabilization happened to have coincided with monopoly formation.
If we had more exchanges we would have more assurance of fair play. Market manipulators would have a hard time running scams simultaneously on multiple exchanges. On a monopolistic exchange that colluded with stealing from traders would be relatively easy. They wouldn't have to worry about the arbitrage bots foiling their scam by trading against the insiders positions on fair exchanges.