Hydro means dams and dams can cause a number of accidents if they are not properly maintained.

Specifically, hydro was the cause of the worst single power generation accident of all time: 171,000 dead when the Banqiao dam failed.

Before anyone assumes that will never happen again: There are fears over multiple large dams, including Mosul, where a failure worst case could kill a million.

If we're lucky Banqiao will be a one-off, but large dams are massive risk factors because they need ongoing maintanenance forever, and are often upstream of some of the densest populated areas. On top of that they're also ecological disasters.

Look at the geography and population downstream of the Three Gorges Dam in China for an extreme worst-case scenario.

Also when they are properly maintained. You don't want to be the maintenance guy getting killed by the combination of high water pressure and a 1cm wide opening.

Here an actuall video on the topic: https://www.youtube.com/watch?v=AEtbFm_CjE0 . It is the pressure difference that results on a large force on any unlucky diver.

In addition to the factors unique to dam failure, large construction projects will statistically result in dead workmen - at least over multiple projects.

Nuclear is a large construction project as well.

Well, really every form of power generation either involves construction or people clambering about on roofs they might fall off of. But this is the reason that forms of power generation with no pollution still kill a certain number of people per trillion Whr.