My understanding was that Google was withholding access to features (vector tiles, turn by turn) to renegotiate maps usage. The two things I heard they wanted - features gated on Google login, and advertisements.

Apple had been developing their own maps solution already and accelerated it to land before the end of their existing agreement with Google, when Apple would have to accept Google's terms to continue usage. Google apparently had no idea negotiations had fallen through until the keynote.

It was going to ship as beta quality anyway (they were merging multiple sources of third party data), but the timeline meant it became a replacement in whatever form it was in.