Why did Darry slap Ponyboy? As a result, what did Ponyboy do?

Johnny wakes Ponyboy up suddenly, and they realize it has gotten late. Ponyboy runs home, scared of facing Darry, but Johnny stays in the lot, since his parents don't care what happens to him. When Ponyboy gets home, Darry is up waiting for him, and Soda is asleep on the sofa. Darry is "madder than I'd seen him in a long time," and he starts to yell at Ponyboy for falling asleep in the lot, saying, "Ponyboy, what on earth is the matter with you? Can't you use your head? You haven't even got a coat on." When Soda starts to stick up for Ponyboy, Darry yells at him, too. Ponyboy says, "You don't yell at him!" In response, "Darry wheeled around and slapped me so hard that it knocked me against the door." The brothers stand in silence, since nobody in their family had ever hit Ponyboy. Darry screams, "Pony, I didn't mean to!" as Ponyboy runs away, back to the lot, deciding to run away.

Why do you think Darry hit Ponyboy?

What is the effect for when Darry hit ponyboy when he came back home.