Could the price drop between July and September 2011 have been caused by a bot?
Since the bot exists, and the people who wrote it presumably wish it to perform every transaction it performs, it reflects real supply and real demand. Why would you want to stop it? There is nothing inherently good about a high price.

Personally, I think the drop in value was due to the bursting of a speculative bubble. The price was previously kept high due to press attention that made people think Bitcoins might be a good investment. As soon as everyone who was going to be brought in by the hype was brought in, the demand started to drop. This caused a small price slide, which induced a lot of panic selling from people who thought the speculative rise would continue forever. Once the price starts dropping at the end of such a bubble, more and more people try to sell out to minimize their losses. Once everyone who expected a short-term profit sells out, the price stabilizes.

We're now back at that relatively stable floor, waiting for the next hype cycle to start another speculative bubble. Until Bitcoin's price is driven by its use as a real medium of exchange, this pattern is likely to repeat over and over.