Aside from spelling and content layout if your hub is too short or does not appear to be attractive/easy to read it might be considered substandard. Examples such as having no breaks or paragraphs between thoughts, having lots of (run on) sentences or a lack of punctuation can also garner substandard notifications.
I suspect those whom english is a (second) language may have difficulty translating their thoughts in a structure that makes sense to those of us which english is our primary language.
Last but not least there are those readers who sometimes read hubs they strongly disagree with or find offensive and they may (flag) your hub for review. This may also happen if the image/photo you used was considered inappropriate as well. The Hubpages team is very responsive to emails and they will tell you what needs to be done if there is a problem with a particular hub.
You might also want to read hubs by those with a Hub score in the high 80s or 90s that write on your subject. They are likely to have several "accolades" attesting to their writing ability. Best of luck!