Or it could be as simple as hauling it off and staggering along on the back of the drag curve with the airspeed reducing until it fell out of the sky.
None of us know the answer but if you want to speculate on why an aircraft within w&b limitations and fully serviceable fails to get away from the field that's one of them. Another would be localised windshear, caused for example by a thermal lifting off as alluded to earlier. In stronger wind conditions it could be turbulence of nearby obstructions that does it. It could be flying too close to the critical AoA so that it stalls in a gust.
All speculation as to the cause of course, and barring any evidence of mechanical failure the AAIB report is likely to to end up offering an opinion as to the factors that may have contributed rather than a definitive cause.