I have had a fair few images rejected for this reason and whilst you could spend hours trying to figure out what may be wrong with your image, it would probably be best to accept this and move on to your next image.
You may find that your submitted image was not what the moderators felt was suitable for thir stock library at that time. If you are consistently finding this to be an issue then perhaps try a different subject and see how you get on with that.
One image declined. Does it really matter?
The grounds might be the quantity of similar pic. In addition, I'd say that the composition could be better. Also the image looks a bit scruffily due to unplesant lens galre and oversaturated.
The fact is that in nature section competition is really tough. You need to be above the average to be accepted and sell good.
However, from personal experience I can tell that some of my pics I considered close to garbage are selling really good. So if you feel your pic deserves more time to spend on it, try other sites.