Also relevant is that Sun had pulled the same trick with Solaris a few years earlier - Solaris 2.6 was followed by Solaris 7. Bigger version numbers make for better marketing. I am skeptical that backwards compatibility was strongly involved.
Apple also did a similar thing with OSX/macOS a few years ago - instead of making everything 10.XX they bump the major version (first number) every year now, continuing on from the 10 that the X represented, as if each version is the same increment as the jump from Mac OS 9 to Mac OS X (which was a jump to an entirely new codebase)
Android did that too, much earlier starting with 5.0. Previously the major version was something of an indicator of a major visual/conceptual redesign. 3.0 was the tablet version, 4.0 was the move to the holo design language, 5.0 was material. Then they just kept bumping the major version every year since.
