My understanding is that the bolts did have castle nuts and retaining wires in the design. So either they were incorrectly fitted, or the bolts themselves were under specified with regard to strength.
Not sure what happened from “the design” to “the field”. The two loosen bolts in the picture are not castle nuts.
Those bolts aren’t drilled for safety wire.
ah. i've been wondering what the method of securing the bolts would have been and have not seen anyone mention it. torque values or threadlocker is rarely enough for the FAA. it's usually safety wire, castle nuts, lock washers, etc.