I have a very long document in a Scrivener project. I was surprised to open the project this morning to find that the document was duplicated (with the -1 appended). But looking at the two documents, it appears that somehow, it just split the document into two parts.
Ctrl+Shift+K or menu option Documents > Split > At Selction will split a document at the cursor. The text after the cursor will be placed into a new document, with the -1 appended.
More than likely you inadvertently pressed Ctrl+Shift+K.