it was all to do with the circulation patterns of 'fresh' air in the cabin.
air was constantly 'renewed' at passenger head-height (i.e. seat level), however not at cabin crew head-height (standing level). this caused the air to 'stagnate' and heat up - or something similar. couple that with the unique environment that is the aircraft cabin: +/- 8,000ft high; dry air and the way the 777 moves during flight, and you got crew fainting every so often on this plane.
BA then tinkered with the airconditioning vents to try to get a better circulation of air in the cabin, to try and prevent this from happening.
they did an article about this in 'cabin crew news' (weekly BA cabin crew news magazine) about 4 years ago.