The answer to this question can be found in the conversation that Biff and Happy together have in their bedroom after they overhear their parents talking. Biff talks to Happy about his life and the way that it has run on a certain repetitive cycle over the last few years. He goes and gets a job on a farm, normally looking after horses, and then when spring comes, he feels an urge to return to the city and somehow try and make something of his life. Note what he says to his brother:
And whenever spring comes to where I am, I suddenly get the feeling, my God, I'm not gettin' anywhere! What the hell am I doing, playing around with horses, twenty-eight dollars a week! I'm thirty-four years old, I oughta be makin' my future. That's when I come running home. And now, I get here, and I don't know what to do with myself.
Biff then returns every spring because he always begins to think of his life and what he is doing with it at this point in the year. He begins to feel dissatisfied with what he has done and achieved and returns to the city to his parents' home to somehow make it big. As the play continues, we see that Biff is just as trapped in Willy's dream of success as Willy is himself, but at least Biff manages to extricate himself by gaining a certain amount of self-knowledge about who he is.