Answer:
Can we take a moment to bask in the joy of how well-written A Raisin in the Sun is... Ok, moment over!
From act 1 scene 1, it's very clear that this family has issues simmering beneath the surface. The question shouldn't be what caused tension, it should be what <em>doesn't </em>cause tension, because Walter and Ruth argue about a large number of things. It's not a happy marriage, it seems.
When Ruth accuses Walter of keeping their son up by talking late at night, he complains, "That's what you mad about, ain’t it? The things I want to talk about with my friends just couldn’t be important in your mind, could they?"
The main issue between them, though, is money. It's underlying everything they argue about - Walter's association with Willy, Travis' ask for the 50 cents, Walter's late-night discussions with his "friends."
