Why is the boss suspicious of George and Lennie in chapter 2 in the book of mice and men?

George does ALL the talking. When the boss asks Lennie questions, George interrupts and answers for Lennie. When Lennie does speak, he merely parrots what George has already said. George explains that Lennie isn't bright, but he's strong and is a hard worker. Their boss asks why George is so protective of Lennie; George tells the boss that they are cousins and Lennie was kicked in the head by horse when he was younger.