When content was first spidered
When a new link was first discovered
Time frames for influxes of links
Time frames for large amounts of content on a specific subject

Changes you make on your site
Changes to search engine algorithms
Changes made by your competitors

You're ranking great, when all of a sudden, a Flickr page or a Technorati tag page or a page at Wikipedia overtakes you. The page is new, has little to no external inbounds, and you're flumoxed by how it can rank well. Don't worry, amigo - that's almost certainly the fresh boost, and it tends to die out after 5-10 days at most.

You're ranking in 10th or 20th place behind some heavy hitters, but your domain is pretty tough and all of a sudden, 5-10 new links point your way. Voila! You're at the top of the results, ranking in front of pages you were sure you'd never overtake this quickly. Once again, it's fresh boost, giving a little bit of "extra credit" to your newfound popularity. I liken this to the search engines almost making the assumption that "whoa! this page got a lot of link love quickly, it must be super relevant/popular for this query, let's give it some juice." The problem is when the engines don't find lots more new links, you start falling down in the results fairly rapidly. Should we call that the "stale drop"?

Rollercoaster mania hits - you're trading places atop the SERPs with 2-3 pages almost every day. I almost want to call this the "tie" flux - new links, and possibly refreshed content on your page and your competitors is making it a really tough call for 1st place, so the tiniest of changes can bump you ahead or leave you behind.