In Shi Shuo Xin Yu, why did Ma Rong decide that he would die when Zheng Xuan "sat under the bridge and exhibited on the water"?

Zheng Xuan studied under Ma Rong, and he didn't see Ma Rong for three years. He was only taught by a talented disciple. Ma Rong used the mixed day algorithm to calculate, but the results did not match, and none of his disciples could understand it. Some people said that Zheng Xuan could do calculus, so Ma Rong called him and asked him to do it. Zheng Xuan solved it with a calculation, and everyone was surprised and admired. When Zheng Xuan finished his studies and set off for home, Ma Rong immediately lamented that the Rite and Music Center would be transferred to the East, fearing that Zheng Xuan would be famous and jealous of him. Zheng Xuan also guessed that Ma Rong would come after him, so he went to the bridge and sat in the water with wooden shoes. As expected, Ma Rong divined the trace of Zheng Xuan on the turntable, and then told people around him: "Zheng Xuan is under the soil, on the water, leaning against the wood, so he must be dead." I decided not to chase. Zheng Xuan was finally saved from death.