Without a signal analyzer with proper demod capabilities there's no way to tell for sure, but my guess is that you've got a room full of people with cell phones in a building with some wireless LANs, all more or less working in the 2.4 GHz band. I've been to conferences with thousands of tech geeks, and while they provide 'high powered' wireless lan (Google conference) it was completely swamped by all the devices.
As it turns out you have a little channel analyzer in that flash, use it to see which channels (if any) are cleaner. I haven't looked at it since I got it, but I think they show which channels are empty
(clean/clear), not which have the most 'strength'.
The other option is to be prepared to fall back to optical if it happens again.
I am a wedding photographer and I have shot 26 weddings this year with three 600ex-rt flashes and for the most part am very happy with them. However, there have been two occasions where there has been an area about 20-30 feet in diamater that the master goes all screwball and the link goes in and out. I was shooting a wedding last night and my assistant also had three flashes and hers did the same thing in the same "dead zone" We were both on different channels so we were not interfering with each other. I did everything I could think of like change batteries turn them on and off and I even did a channel scan to see which one had them most strength.