The reason is an industry-wide requirement set by Apple, Google and Mozilla stating that any two-year TLS certificate issued after August 31, 2020 will be distrusted in their browsers.
As a consequence all Certificate Authorities (CAs) will start issuing publicly-trusted SSL/TLS certificates with a validity period of one year starting September 1, 2020.
To have a unified approach and because one CA is stopping offering 2-year SSL/TLS certificates on August 19, 2020 as well as our upcoming maintenance for our SSL/TLS certificate backend on August 18, 2020 we decided to stop offering 2-year SSL/TLS certificates as of August 18, 2020.
Affected are all SSL/TLS product brands we currently offer: Sectigo, DigiCert, GeoTrust, Thawte and RapidSSL.