They are employees that are not currently happy with the company.

They are x-employees that have quit or maybe fired

They are previous employees or employees of a company that is in some form of reorganization

They are competitors in some form

They have lost money in regards to the investment in company stock

They have friends that have lost money investing in the company stock

They are short sellers on this particular stock

They are related to some type of Market Making firm

They are paid by some unknown firm or individual to ruin the image of the company for various reasons

They are swing trading the moves of the stock-pumping up and bashing down

They are self proclaimed scam busters