This can be for a variety of reasons. The most common reasons for long links being automatically rejected are not adhering to the guidelines in our published Terms and Conditions and/or the Spam Policy.
The link can also be blacklisted if it fails one of our multiple spam/phishing/malicious link checks. Additionally, the link will fail to shorten one if the domain submitted has already been banned on the site for any reason. Finally, if the long link that is submitted to be shortened is not a valid URL, it will fail. These are not the only reasons why you may get an error when trying to shorten a long link, but they do seem to be the most common.