Totally normal practice for major releases during console transitions. Like I said above, the transition form Xbox to 360 is the only console transition I can think of where those weren't common (and that was because MS decided to strong-arm consumers and publishers into the transition to 360). PS3 and PS2 had a ton early in the PS3's life.
You're right that it's a bit less typical for major first-party releases, but there is even precedent for that: most recently, there was a dual release of Twilight Princess on Wii and GC. And it's possible that it's for the same reason. The game may have been originally planned for release on the older platform, but higher-ups decided mid-production to delay it a bit and also release it for the new console.