Arduino Uno SPI CS pin goes low every 20ms

I'm initialising a RFM22 434mHz chip (https://www.sparkfun.com/datasheets/Wireless/General/RFM22.PDF) over SPI using the Radiohead library (http://www.airspayce.com/mikem/arduino/RadioHead/)

It uses SPI and I can see on my scope that all of the SPI pins work as expected, and indeed the chip intialises correctly.

What's odd is if I change the time scale on the scope to say, 10ms per division, I can see the SPI CS pin going low every 20ms. It seems to be regardless of whether the sketch is actually doing anything, and repeats even if the sketch is in the blocking delay() function for several seconds.

My guess is I'm misunderstanding something about SPI, but I would expect it only to go low when the Radiohead library is making an SPI call. Can anyone explain what I'm seeing? Googling hasn't thrown up many results.