question about server lag?

So its a known issue, Evey so often the server (any server) will seem to pause for about .5-1.5 seconds during which client side movement isn’t restricted then every one syncs up and zips to their client side location it seems to happen to everyone. NOW this isn’t a “fix it” post by any means I completely understand that the game is still being developed and this isn’t a request I was just wondering why exactly it happens. I know once in a while you can lag on the EU side but this seems to be server wide, I just wanted to know out of pure curiosity what exact technical factor was doing it, thanks