SD2: The lights rise on the farmhouse, the next day.

SD3: Old Karl still stands in the corner of the stage. He watches teenage Karl walk through the house.
Old Karl: Our story begins on a chilly morning when my mood matched the sunless sky.
SD1: Karl enters the kitchen and sees Pop fiddling under the kitchen sink.
old Karl: Whenevei r we had money troubles, Pop checked the pipes for leaks. You don't disturb Pop when he's working on the plumbing.
SD2: Karl peeks into the room he shares with Theo.
Theo is still asleep.
SD3: Karl walks down the hall to Ingrid's room. She's on the phone and swats him away.
Ingrid: Get out, Karl!
SD1: Karl wanders outside. Mama is tinkering under the hood of the truck.
Mama (to the truck): What's wrong with you now, old-
timer2
Old Karl: You don't disturb Mama when she's talking to the truck.
SD2: Old Karl exits. As young Karl walks around the stage, the scene transforms into a wintry cornfield.
SD3: For a long while, everything is silent except for the crunch of Karl's boots in the snow.
SD1: Suddenly, there is a huge explosion in the sky.
SFX: BOOM!
SD2: Karl freezes. A loud sound punctuates the air.
SFX: Whump, whump, whump
SD3: A shadow passes swiftly overhead. And then, a
gigantic—
SFX: BAM!
SD1: The ground trembles.
SD2: Everything goes quiet. Karl's legs are shaking
SD3: Mama runs onstage
Mama: What happened?
SD1: Karl points offstage.
SD2: Pop appears.
Pops You OK, son?
SD3: Ingrid trudges in.
Ingrid: Karl, what did you do?
SD1: Karl remains too stunned to speak

In Scene 2, Old Karl says, "Whenever we had money troubles, Pop checked the pipes for leaks. You don't disturb Pop when he's working on the plumbing." Why was Pop checking the pipes?

I think that Pop was checking the pipes because he was trying to find a source of the money troubles. Perhaps he thought that there was a leak somewhere in the pipe system and that it was causing a loss of money.

I think that Old Karl is suggesting that Pop was very thorough and that he would not be distracted from his job easily.