I've noticed this very recently and I don't recall reading something about in any of the recent patch notes.
I hop on a flying route and, as any normal human being, get bored of watching the bird flapping its wings all the way until I reach my destination 5 minutes later, so I alt-tab to go browse for something (mostly game-related research, such as finding a recipe/npc location — nothing against game policy!).
Having spent barely 1-2 minutes out of the game, I alt-tab and find myself "disconnected from server". And it's not just once or twice, it really happens a lot and is literally within a couple minutes. Did they reduce the afk timeout or something?
Best Answer
The answer is no. They have not changed anything that effects AFK. Well, not in the sense that you are referring to. It is caused by high server loads for the most part. That and visiting other sites while logged in to the game.
I have the same issue. However, it only effects certain areas. Example, in Deepholm I was disconnected many times during the Stonehearth quest line. I was guarantee to be disconnected if I switch to surfing WoW Head, WoW forums, etc, etc, while logged in to World of Warcraft in the Stonehearth area and the area just to the north of it. That was because of the amount of data that was sent from the server. The server normally sends less than 2KB with few-second-spikes up to 30KB. This wasn't the case in this particular area. I continually had a 15 to 25KB streaming from the server while in this area of Deepholm. Again, it only happens in certain areas. I can be in Org during prime time with slight lag in comparison of my normal lag.
Other effected areas were two spots in Hyjal. One was the area that everyone is first ported to. The second effected area around the Circle of Cinders.
I found few short-term solutions on the official WoW forums from a Blizzard Blue. One is disable add-ons since of them uses bandwidth to talk to other players. An example would the GearScore add-on. Also, discontinuing surfing sites and limit what other system application that uses bandwidth while logged in to WoW. Here is another, which happen to surprise me, disable the your Combat Chat via removing the filters. Here is a link to the post I refer to.