After the boss leaves why is George mad at Lennie?

George and Lennie arrive at the ranch looking for work. Before they meet with the boss, George tells Lennie to keep quiet and let him do the talking. If Lennie opens his mouth, the boss is liable to think he's not all there and so won't hire him—or George, for that matter. If Lennie can keep his mouth shut then, the boss will judge him on his size and physical strength instead of what he says. Once he's been hired and proved his worth on the ranch then whatever he says won't matter anymore.
In selling Lennie's skills to the boss, George mentions that his friend is as strong as a bull. Lennie likes the sound of that and repeats the expression out loud. George gives him a scowl; he wasn't supposed to say anything. Although the boss agrees to take the two men on, George is still annoyed that Lennie opened his mouth when he wasn't supposed to:

So you wasn't gonna say a word. You was gonna leave your big flapper shut and leave me do the talkin'. Damn near lost us the job.