Why was Homer Plessy arrested?

Homer Plessy broke the law on purpose. He was recruited by a civil rights group known as the Comité des Citoyens (Citizens' Committee) to deliberately violate the Louisiana law that mandated separate train cars for white and black passengers. Plessy was a very light skinned man, but under Lousiana law at the time, he was classified as an Octoroon, meaning that he was 1/8th black. It was the goal of both the committee, and of Plessy to bring attention to, and ultimately reverse, the segregation law.
The committee notified the railroad of Plessy's intent to buy a ticket for a white only train car, and he was promptly arrested when he notified the ticket agent that he was only 7/8 white and that he would refuse to sit in the blacks only train car.
Plessy and the committee went to court, claiming that Plessy's thirteenth and fourteenth amendment rights were in violation. They were defeated by Louisiana courts but ultimately ended up at the Supreme Court, where they were also defeated. This Supreme Court ruling established the "Separate but equal" doctrine that remained in place until it was overturned in 1954 following the ruling in Brown v Board of Education.

Homer Plessy, a light-skinned black man, was arrested because he violated the Jim Crow laws in Louisiana. Throughout the South, these laws were passed to keep blacks and whites separated in public facilities. There were separate bathrooms, separate drinking fountains, and separate seating sections on public transportation.
In the situation involving Homer Plessy, he bought a train ticket and sat in a train car that was reserved for white people. When he refused to move, he was removed from the train and arrested. Plessy sued because he believed that his rights, which he felt were protected by the Thirteenth and Fourteenth Amendments to the Constitution, were being violated. In a very famous Supreme Court case called Plessy v Ferguson, the Supreme Court ruled that segregation was legal as long as the facilities were equal. This “separate but equal” doctrine remained in effect until it was overturned in the Brown v Board of Education decision in 1954.
https://www.blackpast.org/african-american-history/plessy-homer-1863-1925/