The speculation I've seen is around a change to the autopilot system which pilots ave been struggling with:

> Following the Lion Air crash, US aviation authorities issued an emergency directive to carriers to update flight manuals with information on what to do when the aircraft’s anti-stall system is triggered by erroneous data from what’s called an “angle-of-attack” sensor. The flight system can react to that data by pointing the plane’s nose sharply downward. Boeing, meanwhile, directed airlines to a checklist in manuals for stabilizing the aircraft. Pilots said the crash and the directives that followed were the first time that they were made aware of these changes to the flight system.