Thanks Vaughn and Gerald, I think you've solved my problem (well, it's not really a problem – it's more of an oddity really).
I make up 33% a Potassium Oxalate solution as standard. Certainly some evaporates while I'm working, although not that much because I keep it in a cafetière which tends to recycle the evaporated water. But my darkroom gets cold at night. I'm not sure how low the temperature gets, but sometimes it's probably below 10°C.
I guess I could eliminate the crystallisation by using a weaker solution, but I'm not sure that I want to do that. So I need to either live with it or keep my developer warm at night.