This message is coming from the new Twitter’s spam protection mechanism. It’s analyzed your message and marked it as spam. As you can expect they don’t share any information about how this thing works. Try to change the format of your messages and use/don’t use/change URL shortener. There are also some reports that Twitter rejects posts coming from hosting providers that were used to abuse it.

Please note: Despite the fact that error is saying “might be automated”, it doesn’t mean that Twitter rejects the post because it was made though the API. “Automated” there doesn’t mean “posted/submitted automatically”, it means “written/generated automatically”.