Great question. This could be the result of using other social scheduling apps or being signed into the same social account across multiple devices.
According to Twitter (but this applies to all social channels), using multiple social scheduling apps at the same time or signing into the same account from multiple devices leads to spam filter triggers. When this happens, Twitter assumes that the last app logged in is the problem and cuts the authentication tokens.
If you are using other apps, we encourage you to disable them by going to www.twitter.com/settings/applications. Here, you can revoke access to social scheduling apps as well as any additional apps you no longer use.
Here is an article (included below) from Twitter with further details on this.
*TO NOTE: This is the case across all social channels, not just Twitter.