Extremely Slow StatMan query issue with #SP2010 farm
This is SQL Server updating the statistics on a table. It can be triggered automatically when about 20% of the data in your table changes, or on demand if you run jobs that update statistics.

You have a few ways to lessen the pain.

You can enable asynchronous statistics updates, which lets SQL Server update stats behind the scenes without blocking other queries. It'll still have a performance impact, though, as Kendra Little explains in The Secret IO Explosion.

You can manually update statistics proactively during maintenance windows. This doesn't stop the chances that 20% of your data will change, but it just reduces the likelihood.

You can disable automatic updating of statistics altogether. This is generally a pretty bad idea since your execution plans may be wildly incorrect, and it doesn't stop folks from manually updating the statistics either.