Usually this is the case when there is already a rewrite present with the same name. For instance if a product has the url: "test.html" and it has a permanent redirect to "test2.html" , trying to add a new product with "test.html" as URL key will not be possible since "test.html" is already redirecting to "test2.html". So maybe the URL is already present in your URL rewrite management?
Please check it out. If you do not longer need the redirect you can remove it and then you can change the url of the category to the URL you want without the suffix (-1 or -2 etc). If the URL doesn't exist (because you previously removed it) then it will be created without the suffix.
note: be careful if the url is already indexed in a search engine and you remove it, this can lead to 404 errors if you do not create a new one.
Hope this helps.