[DonVoigt]

Smaller pieces will corrode quicker, as they corrode from all directions- till nothing is left.
The safety lever is quite thin, so same principle; could also be the pin or the ledge that holds it in could have disintegrated and let the lever "fall" out when touched.

I'm afraid this is another one of those puzzles we will never know the "real" answer to.

But the picture clearly shows that different parts/metals/treatments show different amounts of corrosion when buried. Great picture.