Some competitions have no, or very unclear, statements of how any works you submit to it will be used.
Such competitions will always fail to comply with the standards set out in the Artists' Bill of Rights.
It is possible that such competitions have been created by organisers with no, or very little, knowledge of intellectual property rights. Whatever the reason you should never submit works to a competition which has no clear statements about how they will use your works and how long for.
If you would like to know more about copyright and related rights please read our Guide to Rights and Licensing.