It stopped working for me when I restarted my phone and the app after ending a longish fare. It's possible that the few drivers that were left visible just never closed their app. I was getting pretty annoyed trying to get the app to login, considering the prime timing. But it ended up working out all right for me, because when it finally came back online, pretty much every zone was surging at 1.5x, and I snagged a few before it returned to normal.
It was almost too perfect, and leads me to believe that it could have possibly been malicious. If someone was to launch a successful denial of service attack on Uber and was able to target the servers that the drivers connect to without disrupting the riders, they could potentially create an artificial surge. Kind of like forcing a temporary driver strike to increase demand. This is just speculation, but if people are going to hack, I'd rather have them doing this than modifying their location and screwing over every driver near them.