In Stephen Chow's films, because he was dissatisfied with the attitude of the immortals in the sky towards mortals, he made trouble for the immortals in the sky and caused the jade emperor's dissatisfaction. Under the negotiation between the Jade Emperor and the Bodhisattva, Dragon Lohan was sent down to earth to maintain human order. This is also Lohan's own thoughts and wishes. He hoped that every mortal would be rewarded with one good deed, and evil with another, and everyone should pay the due price for his choice.

The second is the known Chen Musheng version of the Living Buddha Jigong. Luohan, the dragon warrior, accidentally released the imprisoned Dapeng bird and was sent down by the Buddha to hunt it down to offset the mistake of releasing it. While maintaining human justice on earth. This reason may be written by the screenwriter in order to increase the interest and cooperate with the plot of the TV series.

Personally, I think the real reason may be robbery. Just like the Tang Priest in Journey to the West. At a certain level, if you want to continue practicing, you have to go down to the world to survive.