A thread at SERoundtable informs that Microsoft AdCenter takes out the time to explain in detail why certain keywords get rejected.
Reasons cited were:
- Too much ad text (keyword parameter use exceeds character limits)
- Landing page content not relevant
- Website contains forbidden content – Content guidelines
- Incorrect ad format – Style guidelines
- Not enough ad text – The combination of ad title and ad text is less than 6 words
For more read the thread here.