There are several factors that enter into that, according to Google spokesperson, Matt Cutts.
1. Part of it is relevancy to the search query, of course... it seems likely that that's the biggest part, 'though Google's not likely to say too much more than that.
2. Another part is the user's search history. If you've got a lot of searches in your history for African predators, big cats and King of Beasts, and then do a search for the Detroit Lions, Suggest will probably list four-legged lions either above the football team, or instead of it.
3. Anecdotal evidence indicates that personalized search may also come into play now in search suggestions. If the folks you're connected to via social media circuits have a tendency to talk about things that are relevant to your search query, you may find the search suggestions showing their relevant tweets, comments or blog posts above other things that seem more relevant.