Evidently, Facebook changed their algorithm. However, doing so shouldn't have effected your overall Facebook reach. From what we have read, Facebook did this to improve user experience. The issue with brand pages is content. At the end of the day, Facebook will continue to make changes - it's out of our control. What you can control is the quality of your content and you'll be rewarded.
It has also been reported that Facebook is having an issue with Insights data reporting, simply because there is so much of it. No excuses but we are sure they are working on it.
Another theory revolved around end users (fans) are hiding posts - and hiding your brand. Interestingly enough, Facebook decided to 'hide' the 'Hide All' feature, making it more difficult for fans to hide your brand. But to reiterate, they can 'hide post'. Since this is actually a benefit to your brand page, it probably isn't directly related to a lower Facebook reach.
All this said, it's about your content and strategy. Your fans have a lot of control of what content they want to see. Give them what they want and they will be more inclined to read, share and help you build your brand.
