Why are pyroclastic flows ground-bound?
Volcanic plumes and pyroclastic flows are two behaviours of basically the same thing. A volume of hot gasses and tephra (a.k.a. a mix of ash, pumice, and volcanic rocks of different sizes). 

If the gas is hot enough and the tephra content is light enough the mix will be buoyant and it will rise as a volcanic plume. On the other hand if gas is too cold to support the plume, or the weight of the tephra is too heavy. It will collapse and rush downhill as a pyroclastic flow. That is referred to as a "Fountain collapse", and is the most common cause of pyroclastic flows.

That is why the are also referred as density or gravity currents.