It was a major version number change. Per the top of the blog post here, they're switching to a new versioning system with 3.x Prior to 3.x, the major version

It was a major version number change. Per the top of the blog post here, they're switching to a new versioning system with 3.x Prior to 3.x, the major version was the .X part and the 2 was somewhat meaningless. E.g their versioning prior to 3.x is 2.major.minor.patch and will now switch to major.minor.patch