Site link update?

[purple]

Just noticed 3 of my sites have site links for the first time, does google update these on at set times or do they just upgrade sites whenever.

Anyone know what triggers a site link, we have being top of our main keyword for 2 years so maybe thats why?

[MrCatz]

It's hard to say. Some would suggest that this is a sign that Google deems your site to be an authority for a particular keyword.

What are the sites and keywords by the way?