why was the boss mad at george and lennie

The boss is angry because George and Lennie are late. They were due to arrive in teh morning, but because the bus driver lied to them about how far from the ranch he dropped them, the two men had to walk to the ranch. The boss has had to send his labour team out two men down :

'Well, I had to send out the grain teams short two buckers. Won't do any good to go out now till after dinner.'

This will mean he provides a mael for George and Lennie before they have started work. He is not impressed with this arrangement.