The only statement Apple is giving on the matter is:
Apple had originally planned on having the AirPods go on sale in October, but there are unknown issues delaying that. Those who had pre-production models for testing had noted various issues with battery life and physical problems, but there's no knowing why Apple has chosen to push back the release of the AirPods.
Maybe Apple will explain more tomorrow at the Mac event, but we wouldn't bet on it. So, until Apple says more, customers will have to wait longer to drop $160 on wireless earbuds.