Can anyone elaborate on how these situations occur? What happened during the construction of this software? Did someone put in the credentials as a test account

Can anyone elaborate on how these situations occur? What happened during the construction of this software? Did someone put in the credentials as a test account and forget to remove it? Did someone sneak it in? Is it a standard practice? I assume quite a few people worked on that software, I can't imagine a whole department just up and forgot they coded in a backdoor.