Facebook relicenses several projects

Yes, basically that.

It looks like they are dropping an addition/clause to the license - the patent grant - which makes the license incompatible with the Apache Software License.

See: http://writing.jan.io/2017/08/19/understanding-the-facebo... for example, where Roy Fielding of the ASF says he's spoken to Facebook's lawyers and they have said that the BSD+patents license is incompatible with the ASL as written. "In October 2016, I discussed with them my interpretation of the BSD+PATENT addition as a revocation of the traditional BSD implied license and I explained why that would be incompatible with the Apache License 2.0 (as its primary author). To the best of my understanding, everyone on the call agreed that my interpretation was both accurate and intended, and the license has not changed since then. That *does not* mean that FB intended their license to be incompatible from the get go!"

Given that it was incompatible with the ASL that meant Apache projects had to drop it, and other projects (e.g., WordPress) interpreted the license as not compatible with their aims either.