1) What makes that just the last buds, or just the last opened flowers on flower twig fade and die, while others below are apparently keeping health?. (Seen on my Odontoglosum and aldo on Cymbidium.)

2) Whay makes that just the keikis, and not the rest of the plant, show a quick dacay on their leaves, going quickly from normal green leaves to yellow and then fall?: (seen on a my Dendrobium and on my oncidium).