Essentially because Bigger is afraid of robbing Blum's deli and to avoid having to go through with the plan he attacks Gus.
As we learn early in the scene, Bigger is "fascinated with the idea of the robbery, and a little afraid of it." But throughout the novel Bigger does not want to admit his fears and he certainly doesn't want to admit them to his friends. In fact, when Gus admits that he is scared, Bigger seizes an opportunity to exert his power and authority over Gus by teasing him. The plan, however, backfires when Bigger realizes that if Jack and G.H. agree to the robbery then Gus will too and Bigger would then have to either own up to his own fears of robbing Blum or go through with the robbery. So, in order to avoid being put in that situation, he sabotages the entire plan by attacking Gus.
This idea is summed up in the following passage:
Bigger was afraid of robbing a white man and he knew that Gus was afraid, too. Blum's store was small and Blum was alone, but Bigger could not think of robbing him without being flanked by his three pals. But even with his pals he was afraid. He had argued all of his pals but one into consenting to the robbery, and toward the lone man who held out he felt a hot hate and fear; he had transferred his fear of the whites to Gus. He hated Gus because he knew that Gus was afraid, as even he was; and he feared Gus because he felt that Gus would consent and then he would be compelled to go through with the robbery.