It feels (to me), like the CommonMarkdown folk are somewhat frustrated with Gruber.. While he created the initial spec for the format, he hasn't been interested in it for years, and they feel someone needs to own/control it.
By calling the new spec Standard (or common) Markdown, they're implying that they're the rightful team in charge of defining the format, what is/isn't compatible, and how to verify it.
They're treating Gruber very carefully - Giving him Cameos and token references, but not taking his views seriously. It reminds me a little of Stan Lee in the new Marvel movies - He's a quirky old uncle, not someone who would actually make decisions on the ground.
For example - The CommonMarkdown team say they've been working on this for 2 years, but only sent the current name to Gruber two weeks ago - When he didn't reply, they pushed forward anyway. His original license had asked that people don't use the name Markdown for derived works, but this seems to have been ignored.
They also felt that he wasn't a modern or relevant user of Markdown - The team seems focused on people who are using Markdown as a commenting format, more than the authoring format Gruber uses it as.
My guess is that releasing publicly was a forcing function - The team didn't have a response from Gruber, and didn't know if or when they'd get one.
The quote "I assure you that we did not choose the name to make you, or anyone else, angry." seems particularly telling - They didn't write that "We thought this would be OK", or "We thought you'd like this".. Just that they didn't do it with a deliberate goal of making any specific person angry. This seems like a low bar ;)
After they released earlier this week, my guess is that the blowback was somewhat frustrating. They had worked for years on trying to standardize the format, but all anyone was focusing on was the name!
Gruber asked them to shut it down, but (I'm guessing) wasn't interested in an in-depth dialog.
Since they still want to proceed with the project, and (again guessing) still want to be viewed as the default/standard version, they've renamed their project to "Common Markdown".
While they don't have any particular permission for this name either, it has many of the same connotations, but isn't the exact same name. My supposition is that they're betting that since they've now "acquiesced", and changed the name, Gruber will seem unreasonable if he continues to push against them.
Despite having no assurance that the new name is any less objectionable, this lets them concede, and move to PlanB, without risking having to move to a new name altogether.
In any event, as someone without a dog in this particular race, it's somewhat interesting to watch, and try to guess what's really going on ;)