Anyway, many reasons come in mind: boring job, thinking you might steal their jobs, thinking you are terrorist, they are tired from sitting/standing for too long, they see too much people and they are sick of it...
IMO, any job that has no creativity, and you only got to follow the book, and only has to do A though Z... it is boring..
imagine, all day you have to check documents, and stamp stuff... that's it.. no more or less.. same questions you ask, same answers you get.. come on, I would for sure look like someone who been asked to give his two kidneys and a lung..