Is this on all sides of the building?

Do they go through a 'tube' or bracket and screw tightly in to the wood?

They screw into the facia?

The only thing I can guess is the wet / dry = expand / shrink of the wood that they are screwed in to.

Where I live, it would be wind.

No locking device used, screws come out.

Plenty of wind on the side of the house where the problem is worst. 30 feet up also. Wind and thermal cycling were my two guesses. The fascia is covered with aluminum coil stock and determining the rafter ends is impossible. Plus the rafters are mostly 16" on center, but some are less-- as little as 6 1/2 in one case. The original installer spaced the hangers about 32" and used only 1" screws.
Due to wind damage, this one 50' section was replaced yesterday and they (different installer) used 3" screws which mostly went into the fascia board. We'll see how it works out.

Wind, happens only on my western exposure blowing off the lake.

1" screw in soft pine, 50 ft run. You do not see it but but the gutters move with the wood expanding and contracting, the wind, rain in the gutters everything causes movement and the screws just back out from the movement and vibration.