How the hell did they down the plane?

Crab to air missile. Standard.

Or know she was coming?

Craydar

Too many questions.

Are you shore? Don't clam up, I shrimplore you: ask away and I shell try to answer them all.

Coconut crabs are heavy. Heaviest crab on earth. It also climbs trees, the most common is the coconut tree for which it is named. Those claws open coconuts like soft butter, which is specifically why they are called coconut crabs. So, with those details, here's how a coconut crab hurt aviation history:

A colony of crabs climbs a coconut tree. The weight of so many crabs that high in a tree causes the tree to lean, allowing more crabs to climb. Crab critical mass is reached and the tree, bent over like an arch, starts to lose crabs. Being crabs, they drag each other down as a crabalanche clears the palm of all but a lone coconut crab. The tree snaps back upright and hurls the lonely crab into the sky.

Coconut crabs are unable to swim, so it had little choice but to struggle and grab at anything - like a low flying aircraft. Scared, lost, and cold, the crab frantically grabs at anything it can reach, but her aircraft is cloth covered wood spars. Frantic, clumsy claws punch holes in cloth and splinter wood. Amelia has no control as the plane tumbles to earth, splattering across the deserted islands below. She never stood a chance against the crabs.

We got the crabs in the end though. When the South Pacific was chosen wasn't the remoteness of the islands that gave us the perfect target to test nukes during the 50s; it was revenge.

Perhaps one sneaked on the plane...

edit: Crabs on a Plane?