tl;dr; They changed to matomo so that they can register the trademark.
I'm a heavy piwik-user an I love it. But this name-change sucks for me. Especially because the mysql-tables where prefixed with 'piwik' which will now be confusing.
I might be wrong, but after reading their official announcement, they clarified that nothing will change apart from the project's name.
The rest of the code will remain the same.
For 10 years they have been using the same code with the same name and I don't think they will rename their prefixed tables with the changed name for obvious reasons.
I'm a heavy piwik-user an I love it. But this name-change sucks for me. Especially because the mysql-tables where prefixed with 'piwik' which will now be confusing.