The announcement: https://www.elastic.co/blog/licensing-change
The "Why" behind the change: https://www.elastic.co/blog/why-license-change-AWS
tl;dr Elastic alleges that Amazon is infringing on their trademark and is offering Elastic's products as a service on AWS without being a good partner.
From Elastic's summary of the license change: "The SSPL allows free and unrestricted use and modification, with the simple requirement that if you provide the product as a service to others, you must also publicly release any modifications as well as the source code of your management layers under SSPL."