Since it isn't clearly mentioned on the site: PatternFly is founded by RedHat (now part of IBM) and serves as a foundation for https://ux.redhat.com/.
Formerly PatternFly was rather RedHat specific but it looks like they've spin-off the RedHat parts and made PatternFly general purpose (which is great ofcourse).
IIRC, Patternfly started many years ago, because many open source project (of Red Hat interest), had inconsistent ui. The idea was to develop a common guideline that other open source projects could use and be somewhat consistent with each other. To have a framework, artwork and guidelines that would be available on permissive terms forever.
Formerly PatternFly was rather RedHat specific but it looks like they've spin-off the RedHat parts and made PatternFly general purpose (which is great ofcourse).