Why did Gao Yan infect his brother Gao Zhan?

During Xiao Zhao's reign, he was both civil and military, worked hard politically, appointed many talents, and made great contributions to national construction. Politically care for the people, reduce taxes, send inspectors to observe the people's sufferings, formulate national policies, and reduce the people's burden. However, in the second year of Gao Yan's reign, Gao Yan fell off his horse and was seriously injured. The Queen Mother often visits the sick and asks about the whereabouts of Yin Gao, because Gao Yan agreed with the Queen Mother not to kill Yin Gao. The Queen Mother finally found out that Yin Gao was killed by Gao Yan, and she was very angry. Gao Yan regretted it at the end of his life and begged his mother's forgiveness, but in vain. In the same year, Gao Yan died, only two years in office, at the age of 27. In order to protect his son, Gao Yan passed the throne to his younger brother Gao Zhan, and asked Gao Zhan not to kill his wife and children by writing a book before he died. In 564 AD, Gao Yan's son Gao Bainian was killed by Gao Zhan at the age of 14.