I am assuming it is still in top 10 results? If so, there's no need to worry about it too much.
Edit: As you know, snippets are shown at Google's discretion.
Google re-arranges search results for a lot of reasons. Here are some:
1. You searched once, and then searched for the same term again. Google decided the first time around, you didn't get what you wanted (because otherwise, you won't search for the same query, would you?), and so, re-arranged the results for you.
2. A part of the search results is always under "test" for validating updates to existing algorithms. Your search query and its SERPs happened to be part of that.
3. Google personalized it based on the first "you" and the "second" you. (Some possible factors it takes into account: Sites previously visited, location, browser, connection speed)
4. Multiple data centers are serving the results for your query and so, the SERPs are different.
5. Random shuffling of results is done for certain queries, when Google decides it needs more data. This could be the result of that.
So, as long as you didn't build any bad links to the page, or change the page's internal structure too much, you should be good.
I am sure you are watching the number one result in SERPs to make sure you are not missing out on anything.