Google has no comment on why many webmasters have seen their Panda 4.2 gains reversed on August 14.
Google Panda 4.2 was released by Google as a really slow update that started on July 18, 2015. But around August 14, some are speculating (including myself) that the Panda 4.2 update was reversed or changed significantly.
We knew that Panda 4.2 would roll out over several months and that over time, more and more pages on your site would be updated with the revised Panda score. So if you recovered from the Panda penalty, you should have seen a small and gradual improvement in your Google rankings as the weeks passed. But after August 14, many of those who saw the small and gradual improvement noticed that those gains were wiped out overnight.