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 doesn't 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."