Not Lag exactly. It is, as I understand it, the way the game handles the massive amount of information for all the players in the game. IIRC, it works something like this. There is no way and no reason for your computer to know the specific actions of planes and tanks on the other side of the map. The only that is relavent to your computer is what is directly around you and maybe within a specific set distance/sector. The server sends you the packets of information related to all the other players within that specific (but small) distance.
Now, having that information in mind, people have been known to break spawns (especially at hangers where your position is known), by going to the hanger of a base on the other side of the map. They can then select a tank (preferably with a fast turret and hard hitting round T-34-85. Then, in very rapid succession, jump back to the tower of the base which you are camping, spawn in the hanger and begin moving their turret. The server in turn takes time (a sec or two) to update the spawner's new location on the map. It then has to begin sending out the spawners packet information to all the other players in the area. This also takes a second or two sometimes even a bit longer. In the mean time, the spawner, who already knows your position, is turning his turret toward you, even though your computer does not even know he is there. He then fires a single round into yoru tank at point blank range, sometimes even before your computer knows he is there. Because your computer has not received 'his location' it does not display on your screen, even though you are displaying on his. You have now died even before the enemy tank 'fades in'.
Essentially, it looks like you died from an invisible tank. When in actuality, he was there, but was exploiting the time lapse to send information from his computer to the server and back to your computer.
Now that it has been explained, before you say anything about 'cheats', 'gaming-the-gamers', 'exploiters', you will not get simpathy from a lot of players (including me) over losing your spawn camp. I don't mean this disrrepectfully toward you personally, I just think hanger camping is by-and-large a weak form of play. So, if one is going to do it, I don't have a problem with people taking advantage of an 'exploit' to break it.
Sorry, but that is just my opinion. <<salute>>