This video shows a very sudden flush by the loon sitting on the nest. Normally, we would see a head down/neck extended posture or some other indication of alertness before a loon flushes from a nest. And rather than a wail after leaving the nest, we would hear either a tremolo or a yodel, vocalizations more indicative of alarm.
It appears that the loon’s eyes were looking almost straight up prior to flushing. Could an eagle have flown a little too close to the nest? We’ll probably never know.