Some just prefer to reject because they know there is nothing amazon can do about it (unless you are reported), and it is unlikely they will be surveyed.
There are also some HITs that farm out the same task to a number of workers and then take the answers given by the majority of the workers to be the 'correct' one but sometimes there just isn't a clear cut answer and what the other workers would put up is anyone's guess. The Are these items different HITs are the best example of this though there could be others as well. There'd also be instances where one guy conscientiously follows the instructions but the others who're not working on don't and they come up with different answers and it's the conscientious guy who gets rejected.
As noted above, requesters should provide some reason why they rejected you HIT. This would greatly help us in the future to do better with their requests (and provide some measure of closure so we're not left wondering what went wrong).
This is a huge problem on mTurk and other related sites. I guess the only thing we can do is to simply do HITs and hope we'll get paid. There was one guy once who rejected a large amount of HITs I did for him, even if they were done properly. But I reported him and he got banished. I recommend you to do this, too.
Some Requesters do reject HITs intentionally though there's nothing wrong, however, many a times, while we feel our HIT was wrongly rejected, usually there may be some mistake made by us which we may have not noticed or which we aren't eligible to notice. For example, take the first post itself here. While the one asking may seem there's nothing wrong, we can see the spelling of requester, if missing after even, a space left between right and comma (,) which shouldn't have been (I'd prefer fullstop to comma here actually); and no question mark after please, that is, at the end of the question!!!!!!!!! Some Requesters are very strict for spelling and grammar mistakes. And for someone who doesn't have enough knowledge of all this won't find any mistake therein. Ignorance is bliss, but that's not always
One thing is for sure, if you do not have a mastery of the English language, it's probably best not to accept hits that require writing.
Quoted for truth. Requesters really should start using qualification tests more to weed out unsuitable workers. It's too bad some workers can't be trusted to weed themselves.
Here's the thing, though: There's the text book way that foreign languages are taught and then there is the actual way the language is spoken with a natural flow. I can speak and read spanish well enough to survive, but I would never accept a hit that required translating english to spanish and vice versa because I'm not familiar with the "slang." It's easy to see from the posts here that many people have a really good grasp for casual conversation and posting; but not for requestors who are looking for precision. English is a really hard language to master. I'm really grateful that it's my first language!
If a requester is ethical he/she will respond. That's the problem with those who aren't. They won't respond period. So when you know you've followed the hit instructions to a "t" and know they were indeed correct, well, you know you've been scammed. It's the internet. Fortunately most of the requester's are ethical.