When considering the occurrence of rarities in HK, I think we should not focus on one assumption first.
We should explore reasonable theories and pick up the likely one to further investigate.
If you have an assumption first, it is easy to have an answer before any examination of facts and observations.
My initial thought is wandering of young or immature birds. Pls note that some of the birds you mentioned are large waterbirds. their range of movement is really great and out of expectation.
