The definition of vehicular manslaughter is unlawful or negligent operation of a vehicle resulting in a death. If the driver was not driving unlawfully, such as DUI, then it is not manslaughter. Fleeing the scene of an accident is unlawful so that also makes it meet the definition. Other reasons could include speeding or running a red light.
That still leaves us with the second part - "...or negligent". I find it a bit hard to believe that in all those cases the cyclists were at fault? Unless you consider that they were already acting recklessly because they tried to use a bicycle on a public street (I know a lot of people subscribe to this opinion, but I don't).
I don't want to be too insistent, but: if you get hit by lightning or a falling rock while cycling or driving along, that's no-ones fault. But as long as two vehicles are involved, there is a set of well-defined rules that are designed to make sure that these vehicles don't collide, and if they do collide, in the overwhelming majority of cases one (or both) of the parties involved has failed to follow these rules, i.e. negligence.
https://www.law.cornell.edu/wex/vehicular_manslaughter