I can certainly believe that there are plenty of people that try to manipulate the stock price, but most Apple haters aren't playing the stock market.

By far the most common cause of Apple hate I see is what appears to be consumers who are essentially disgruntled that none of Apple's products are targeted at their own use case: eg. gamers who always decried the Mac Pro because they wanted a cheap traditional desktop, not a workstation; or people who wish Apple was competing in the $500 laptop market, or the 8+lbs mobile workstation or gaming laptop market. A large portion of complaints about Apple hardware being overpriced can be traced back to misunderstandings about what Apple's actually selling and how it differs from the Dell that somebody tries to compare it to. (Of course, there are plenty of things that Apple does over-charge for.)

Next most common are probably the people who despise the "walled garden". They're quite vocal in these parts of the web, but I doubt they outnumber the previous category of Apple detractors.

Or people who don't like having to deal with dongles.

After seeing Jim Cramer mention on camera the idea of putting out a false Apple rumor (example given in 2006 was "The iPhone is being delayed"), I'm less inclined to write off conspiracies in this regard. Don't know that it's a full-blown Conspiracy™, but the financial incentives are there for the right people.

The financial incentive is much more pedestrian: hot take clickbait for struggling news organizations.