This is what I found regarding how it happened, but still not a full explanation - but there was some name confusion and obvious incompetence.
>According to eMudhra, proper strong keys were created in the HSM at the key ceremony on 30-04-2025. The script had been changed to test wrapping and unwrapping keys, because the intermediates were to be moved to data centres in the US and Europe, and a bug in that change created a second set of weak keys. The keys had almost identical names, and the weak ones were used for the certificate requests. The same request was signed by eMudhra's own root and sent to Sectigo, which cross-signed it. The strong keys never made it into any certificate (Bugzilla).
>According to eMudhra, proper strong keys were created in the HSM at the key ceremony on 30-04-2025. The script had been changed to test wrapping and unwrapping keys, because the intermediates were to be moved to data centres in the US and Europe, and a bug in that change created a second set of weak keys. The keys had almost identical names, and the weak ones were used for the certificate requests. The same request was signed by eMudhra's own root and sent to Sectigo, which cross-signed it. The strong keys never made it into any certificate (Bugzilla).
https://www.fairssl.se/en/articles/emudhra-ca-noegler-knaekk...