Meaning of to catch someone with their guard down

The phone immediately rang and Sam stared at it for a minute. Should he let Josh get the satisfaction of the last word? He looked at the shattered remains of his television, picked up the phone, and shouted, "Look, you wormy little fuck, you're lucky I don't come down there and pop your head like a pimple!"

"Sam, this is Julia, down at the office. I have Aaron on the line for you."

"Sorry Julia, I was expecting someone else. Hang on a second." He sat down on the couch and held the receiver to his chest while he tired to regain his composure. Too much change, too fast. He couldn't let Aaron catch him with his guard down. His good friend Aaron, his partner, his mentor.

Thanks so much.