King Xiaowen of Qin had a very brief reign. Indeed, he only became King of the Qin

Qin (state)

The State of Qin was a Chinese feudal state that existed during the Spring and Autumn and Warring States Periods of Chinese history...

 for less than 1 year, and 3 days after his coronation he died.

This gave rise to many theories as to his short reign. The most accepted one is that he was very old when he ascended to the throne (his predecessor, King Zhao of Qin, ruled for over 50 years).

However, there is a conspiracy theory: that Lü Buwei

Lü Buwei

Lü Buwei , Lord Wenxin 文信侯 was a Warring States Period merchant who schemed his way into governing the State of Qin. He served as Chancellor of China for King Zhuangxiang of Qin, and as regent and Chancellor for the king's young son Zheng, who became Qin Shi Huang, the first Emperor of China...

 poisoned the king or at least hastened his death, to put the next king, King Zhuangxiang of Qin

King Zhuangxiang of Qin

King Zhuangxiang of Qin , personal name Yiren , later renamed to Zichu , was the ruler of the Qin state during the 3rd century BC in the Warring States Period of Chinese history.-Biography:...

 onto the throne. This is supported in a way by the fact that Zhuangxiang reigned for only 3 years.

So some people link Xiaowen and Zhuangxiang's deaths all the way to Lü Buwei, claiming that it was a plot by him to put his son, Qin Shi Huang

Qin Shi Huang

Qin Shi Huang , personal name Ying Zheng , was king of the Chinese State of Qin from 246 BC to 221 BC during the Warring States Period. He became the first emperor of a unified China in 221 BC...

, onto the throne.