Its not about startup or autostart.... You might have some application that is installed on your phone and you would have mistakenly given the right to run... Not an issue... just see the application name in Setting>Apps>Running and uninstall/Disable. Or if you want to do another thing just force stop and then clear application data fully. Then application cache will set to defaults. And you have give any right to execute it will ben revoked and your problem is resolved without harming the parent application.
Another thing is that, you cannot start an application natively, automatically... But yes there are some few apps in Play store that will automatically launch that application at scheduled event.
Note: The launching of application is not same as that of the application running in background (background service). Background service is meant for some specific task to be done automatically that generally do not need any user input/intervention. You can launch application at scheduled time either by programming or installing third party scheduler app that will meet your needs. But you cannot start application's background service..... And technically its not of worth for you.