To "explain the circumstances" means to provide a detailed account or description of the situation or events surrounding a particular incident or situation. Full definition
It should explain the circumstances in which it is being filed. (supremecourt.uk)
In fact, as it stands, this is quite correct, for one has not really understood something in its entirety simply because one knows how to explain the circumstances surrounding its beginning. (firstthings.com)
But it's important to explain circumstances under which the Code allows them not to be. (realtormag.realtor.org)