Apparently, two redundant systems failed at the same time.
On September 24th, many Gmail users received an unwelcome surprise: some of their messages were arriving slowly, and some of their attachments were unavailable. We’d like to start by apologizing—we realize that our users rely on Gmail to be always available and always fast, and for several hours we didn’t deliver. Google Apologises Over Gmail Outage [gmailblog.blogspot.com]
The two network failures were unrelated, but in combination they reduced Gmail’s capacity to deliver messages to users, and beginning at 5:54 a.m. PST messages started piling up. Google’s automated monitoring alerted the Gmail engineering team within minutes, and they began investigating immediately. Together with the networking team, the Gmail team restored some of the network capacity that was lost and worked to repurpose additional capacity, clearing much of accumulated message backlog by 1:00 p.m. PST and the remainder by shortly before 4:00 p.m. PST.