If I'm not mistaken: The two projects were started by the people that founded Element. When they founded Element, they transferred the projects to the Foundation, to make them more independent. Now, they're taking them back and relicensing them with a reciprocal license (AGPLv3 + CLA) instead of a permissive one (ALv2). I assume this is about other companies using Synapse and Dendrite without contributing back, so now Element wants to be able to sell licenses for the projects, or get contributions. See
https://element.io/blog/element-to-adopt-agplv3/ for more details.