Is it because they know that they can't be caught up to?

I hope this isn't a reason they do, but more a lack of a reason not to. They're total expletives for throwing things, but I'd really hate to believe they do it only because they can't be confronted about it. They may well believe (explicitly and consciously) that roads are intended only for motor vehicles so pedestrians and cyclists are tresspassing.

Note that dangerous ignorance occurs in cyclists as well: I once encountered a cyclist who firmly believed that he had "the same [rights] as a pedestrian" (if so I wonder why he signalled his turns) - he'd just nearly caused a collision by making a (signalled) turn across traffic while ignoring a stop sign(!) for his direction. Such negligence angers compentent users of the roads, and helps link "non-motorist road user" to "danger to self and others" in the eyes of the motorist.

These don't excuse the throwing of stuff, however I wanted to explore some potential reasons for a strong negative response to non-motorists - strong emotions cloud judgement, as in road rage.

When people tell me they need to commute in a truck for "intimidation value", I tell them I need to cycle carrying an RPG for intimidation value, but the government won't let me buy one.