I’ve heard a couple of versions - both similar - from United 777 pilots. It has also been reported in the media both pilots received additional training, which points to human error. It appears this error lead to an undesirable aircraft state. The investigation will reveal the “why”. My hunch, as with most incidents involving highly trained and experienced crews flying very capable aircraft, is it will involve multiple links in the error chain, to include human factors, recency of experience (the FO was reported to be on IOE), procedural errors, non-standard takeoff configuration (word is it was a flaps 20 takeoff), environmental factors (heavy rain/turbulence), and fatigue.