Finally Google have explained the new site links we have been seeing over the last few weeks:
For the two line format:
Ad Sitelinks are designed to trigger in situations where an ad provides the ideal answer for a search query – i.e. for exceptionally high quality ads. These ads are most likely to trigger on unique brand terms.
For one line sitelinks:
Ads will trigger with slightly more generic terms, but may also include brand terms. These are for high quality ads and can appear for multiple ads on the same page.