Do locals train hop?

I notice more than a few times that my seat which is reserved by me is sat by someone else. The moment the see me they move to another seat. I wonder how common is train hopping in Taiwan?

People can buy unreserved tickets - these let them take unoccupied seats, but if somebody (like yourself) shows up with a reserved ticket, they are required to move. If all the seats are reserved, then they must stand.