Reason for Whatsapp outage?

Whatsapp with more than 1.2 billion users experienced a 2 hour outage on wednesday. Many journalist, activist, etc. use this service and it's encrypted communication. Over the alternative of local mobile carrier text messaging. Their explanation for the outage: "Earlier today, WhatsApp users in all parts of the world were unable to access WhatsApp for a few hours. We have now fixed the issue and apologize for the inconvenience" Anyone else have any info on the cause? Hacked? Genuine server issues? other?