Toppled Hive

I decided to pop round my apiaries today as it had been over a week since my last visit and I'm so glad that I did. A hive was toppled over and judging by the grass being yellow underneath where it had fallen it had been like it for a time. I could see the bees clustered in the frames so I carefully reassembled it. It will be Spring before I can see if all is well! I am wondering how it happened; the stand was fine and where it was situated, if it had blown over others would have fallen too.