Lots of reasons for bashing, but all centre on self interest......Which is the better computer, you ask a sales associate...."Definitely this model here" you're told as the sales person points at one particular machine.
Why that one? Chances are there is an increased commission or other type of incentive to get sales staff to move that particular model. So you buy it, thinking that the salesperson has your best interests at heart. She or he doesn't, in most cases, they're doing what is best for them and not for you.
What does that have to do with the stock market? If you participate on a message board, everything. Sharaeholders will gather on a board to talk about their investment, and being shareholders they'll tend to be positive in most cases, happy with their decision to buy in to a particular company.
But excited stockholders might have to contend with others who will do everything they can to point out the negatives, and will often suggest selling. "Get out now while you can, before the big drop". That's pretty common language from posters typically referred to as bashers.
But why the bashing? It may be that the bashers are attached to short interest, meaning they've sold the stock in hopes that it will drop, with hopes of buying it back at a lower price. Getting the price to drop and then buying back requires that shareholders be willing to sell, most often at a price lower than what was paid.
In other words the basher is working in his/her own self interest, no different from the sales associate in the computer example.
There could be other reasons, especially in industries which are competitive. Start up companies for example often rely on the captial markets to raise funds in the hopes of bringing a product or service to market. If there are other companies in the same business space, its logical that less competition is better for their bottom line. Again, self interest.
But whatever the motivation, be it short interest or some other desire to damage the targeted stock, realize that it is not with the best interests of shareholders in mind.
That is not to say bashers are not sometimes right, and that shareholders might do well to heed their advice on occassion by selliing to either lock in gains or to limit losses. Just as with the computer example, sometimes the product being sold has the highest commission AND is the best fit for the consumer.
But not very often.