Good, fairly in-depth (for a 'civilian' newspaper) article on the suspected cause:
NTSB probe of Whidbey floatplane crash points to likely cause, fix | The Seattle Times
Videos of lab tests conducted by the NTSB released Friday show that the lock ring securing the two parts of the actuator can disengage when the clamp nut at the top is tightened.
Also an interesting 'belt and suspenders" fix being implemented by
Friday Harbor Seaplanes in the aftermath.