My understanding from DigiCert is the cross-signing with Entrust had been done awhile back to improve mobile browser compatibility. Perhaps this is some strange combination of developer tools installed and the platform they are developing for...
The most obvious answer would be that it's being installed by some widely-used piece of software, but I don't know what.
Pity the "Date Modified" column is empty, and I don't think there's really a log of what added things to the keychain.
We've worked around the issue for now by not using EV certificates, which isn't a great solution.
Could be just about anything. In my case, my keychain has followed me from one Mac to the next since before the cert that expired today was ever issued, so it could have ended up in there anytime in the last ~8 years from anything I might have had installed dating back to my PowerBook G4...
Virtualization software might be a candidate.
Actually, I just had another thought: Steam. And when I just tried to go to https://store.steampowered.com/, guess what certificate is in the trust chain?
Typically servers will present their certificate and intermediates but not the root, under the assumption that browsers must already have the root in their CA store. So for DigiCert that would probably be all the certs up to but not including "DigiCert High Assurance EV Root CA".
You can see the presented cert chain using `openssl s_client -showcerts ...` or the Certification Paths section of the Qualys SSL Labs Test: https://www.ssllabs.com/ssltest/analyze.html?d=github.com
Do you see an expired "DigiCert High Assurance EV Root CA" certificate in your login keychain? If so, delete it. If not, something weirder may be going on.