According to Wikipedia, Bruce was arrested in 1958 due to a fight with a rival school's student, and as a result, his parents decided it would behoove him to return to the United States (emphasis mine):
In 1958, after students from Choy Li Fut, a rival martial arts school, challenged Lee's Wing Chun school, he engaged in a fight on a rooftop. In response to an unfair punch by another boy, Bruce beat him so badly that he knocked out one of his teeth, leading to a complaint by the boy's parents to the police. Lee's mother had to go to a police station and sign a document saying that she would take full responsibility for Bruce's actions if they released him into her custody. Though she did not mention the incident to her husband, she suggested that Bruce, being an American citizen, return to the United States. Lee's father agreed, as Lee's college prospects were he to remain in Hong Kong were not very promising.
In 1959, this idea was solidified and Bruce was sent stateside:
In April 1959, Lee's parents decided to send him to the United States to stay with his older sister, Agnes Lee (李秋鳳), who was already living with family friends in San Francisco.
However, further reading suggests that Lee was no longer studying under Ip Man in 1958:
Lee was 16 years old under the Wing Chun teacher Yip Man, between late 1956 and 1957, after losing to rival gang members.
So with that in mind, why did Bruce Lee's studies with Ip Man come to an end?