insert tab changes paragraph style

Frequent problem, here. I've modified several built-in heading paragraph styles. I've created a few new paragraph styles, some of which have custom tabs (left and right align). The new paragraph styles are based on: no style; they aren't associated with any list styles; they aren't added to template. But, for unknown reasons, sometimes inserting a tab character (or, pressing tab key anyway) causes these paragraph styles to change into heading styles, rather than to insert an actual tab character. It seems like Word 2011 thinks I want insert tab to result in increase outline level, as it would if I were working in an outline list, even when I'm just hitting tab in a regular (non-list) paragraph. This happens in both outline view and in print layout view. Where is it coming from?

Thanks.