The cause is a 2FA fail with either SIM security or a mobile number port-out scam as the point of failure.
Brian Krebs writes for KrebsOnSecurity:
Of particular note is that although the Reddit employee accounts tied to the breach were protected by SMS-based two-factor authentication, the intruder(s) managed to intercept that second factor.
In one common scenario, known as a SIM-swap, the attacker masquerading as the target tricks the target’s mobile provider into tying the customer’s service to a new SIM card that the bad guys control.
Another typical scheme involves mobile number port-out scams, wherein the attacker impersonates a customer and requests that the customer’s mobile number be transferred to another mobile network provider.
Were you exposed?
...between June 14 and 18 an attacker compromised several employee accounts at its cloud and source code hosting providers. Reddit said the exposed data included internal source code as well as email addresses and obfuscated passwords for all Reddit users who registered accounts on the site prior to May 2007. The incident also exposed the email addresses of some users who had signed up to receive daily email digests of specific discussion threads.