I had two fall over in the last gales a few months ago. They were both freshly filled, the contents hadn't properly settled and compacted at the bottom of the dalek yet. It happens.
At the moment because it is so warm, the contents shrink rapidly and get topped up with weeds. I don't turn the bins over at this stage or they would be less stable again. Did your neighbour turn his dalek recently?
Are you on clay? Clay does shrink quite a bit in this heat. The base of the bin may no longer be level due to shrinkage.