As someone who has owned two commercial flat roof buildings I cant stress enough that you MUST do inspections of your roof at least twice a year. Especially if you live in a big city. I've had backups caused by kids roofing balls and bottles, stolen purse, dead squirrel, dirty balled up diapers from the neighboring apartment building. City living for ya.
Yeah, I'm pretty sure in this case it was a combination of having a 4ft parapet around the entire roof, and having basically never done an inspection. Not enough drains and they were all full of leaf matter.
