Germany just beat Brazil 7-1 in Brazil, and Brazilian fans are understandably unhappy about the result of the game. The crowd in Belo Horizonte was also very heavily Brazilian, with German fans who got their tickets through normal means concentrated in one section. In an attempt to avoid potential altercations, this happened:
PA told German fans to stay in their seats so they could be escorted out. #run— Graham Watson (@Yahoo_Graham) July 8, 2014
If this sounds abnormal or particularly ominous to you, there's no reason to be alarmed. This is standard procedure around the world for away fans, especially for rivalry games or very important games. Away fans are kept in one section, police and security wait for all the home and neutral fans to clear, then away fans are escorted out of the stadium to their transportation.
That's not to say that this isn't likely to get ugly, but this isn't abnormal. It's just a common-sense safety procedure.