Viruses can spread quickly on any cruise ship.
It's not news a ship suffers from them. What is news is that not everyone aboard catches it. 400 out of 2500 passengers is one in six. Without the crew's efforts to minimze the spread, three, four, five, or six of six passengers will catch it.
The early reports hinted the first passenger with the illness had symptoms Monday night, the first night of the cruise. Since the Norovirus, if it was a Norovirus, has a 72 hour incubation period, that means that someone(s) came onboard already sick.