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?