Just because it's a noisy signal, doesn't mean it's not a signal. It has something to do with Canon, and where there is smoke there is fire. If the 5D3 was killing it, I doubt we'd see Canon's stock fall off so dramatically against the benchmarks. And if you look at the chart, the slide begins right after the release of the 5D3.
Either way, a stock price falling is a sign of trouble for a company. If a company is making bad decisions overall, there is a greater probability that any one decision, no matter how small, was bad.
Stocks are a really tricky thing though. A lot of industry is cyclical, and ironically the DSLR industry tends to be cyclical in overlapping generations for one brand or another. Canon is currently in a downtrend right now in their cycle, while Nikon is currently starting an uptrend. Such statistics don't always stay the same, and cycle period can change quickly. But things aren't that simple, either. Canon has had a lot of other factors to deal with, from natural disasters to recent closures of plants because of things like riots in China. Canon is very sensitive to their market share, and interruptions to production, even minor ones, can mean a fairly significant hit to their bottom line...and the bottom line is one of the most significant effectors of stock price there is as it affects various other statistics and indicators, like P/E ratio, dividend and yield, etc.
On that note, and to the original comment that started this tangent on stock prices...I don't recall the 5D III having particularly lackluster sales at a $3500 price tag. As far as I saw, it was flying off the shelves as fast as the D800, and more of them sold than the D800. I believe part of that is due to the fact that Canon tends to have a stronger supply chain than Nikon, but the 5D III has been extremely popular among wedding photographers. I wouldn't be inclined to point to the 5D III as the sole cause of Canon's declining stock price. Canon is a huge company, and while they may not be as large as Sony in sheer scale, they have their fingers in a LOT of markets related to imaging, photography, optics, and a variety of related electronics and devices. Any one of the major sectors of their extended empire could be affecting their stock price, or many, or all of them. Here in our little world on CR, we see Canon as lagging behind a bit technologically speaking. I'm not really sure thats how the broader world sees them.
As someone who lightly trades stocks myself, Canon is one of the stocks I watch. Most of the stock news related to Canon is more broad-market related, japan index related, than specifically related to the performance of the company itself. I believe Canon is a major company in Japan's major indices, so much like similar companies in the US it can ride the index waves up and down. Lot of pressure on Japanese stock indices lately. Most of the news related directly to the performance of the company seems to be generally good, including a variety of M&A news. However mergers and acquisitions can cause odd behavior in a stock, as people trade in or out of their positions in lieu of share swaps or purchases, dilution of stock, etc. (although I'm not sure any of the M&A news for Canon is really serious.)
Anyway...trying to guage a company like Canon in the context here on CR can be a really tricky thing. Hundreds of factors affect a stock price for a company like Canon. I wouldn't read into a 30% decline in price as solely due to what some may consider a lackluster release year or a presumably high introduction price. Canon still has a LOT going for them, and overall I would call the 5D III and 1D X strengths (yet to see how the 6D fits into things...that one might indeed be a dud!