Yep it’s a curious effect all right. On twitter I’ve been presented with many explanations and reasons. The first night I was observing it was so windy I elected not to use the DSLRs for this very reason, 2nd night (peak) was much less windy although there were some short spells of gusty winds and the 3rd night was fairly windy again (but I haven’t got to checking all of the third night images yet!).
What would clinch it would be some double station imaging with suitably placed cameras. I’ll just need to go back to do some more imaging…. 😉