One of the most interesting things about the fact that we’re seeing Olaf’s Frozen Adventure attached to Coco is the fact that this is the first time that a Pixar film will have a piece Disney Animation Studios attached to it.
The more likely scenario, however, is that Disney didn’t want to wait until next holiday to get Olaf’s Frozen Adventure out, and since Disney doesn’t have any animated films being released this year, they decided to take the spot from Pixar.
It’s possible that Pixar just didn’t have a short that was going to be ready in time for Coco’s release.
Previously, every Pixar film that has had a short in front of it, which is nearly all of them, it has been a Pixar-produced short.