They have details and an explanation of this change here:
Since I provide an MIT licensed project with a significant dependency on TinyMCE this has quite a large impact for me, and I imagine there could be quite a few other projects in a similar position since TinyMCE is quite popular.
I respect that the GPLv2+ will ensure greater freedom of code to users, so should be a good change long term, although the reasons given are all quite commercial-focused, and I have recently seen tiny move features from their open offerings into their proprietary additions. They also (in my opinion) misrepresented their previous LGPL license to enforce branding requirements, which I used as the second example in my blog post here.
It’s great they’ve kept it on a FOSS license though, rather than switch to a source-available/proprietary license.