The most important factor that contributed to their survival was probably the fact that there was a steep slope outside the windows, hence they did not fall on flat ground but instead rolled down the slope which slowed their fall and thus softened the impact.

Furthermore it is possible that the ground at that time of year was covered with thick undergrowth, which could also contribute to their survival. Also it is worth noting that the 17th century clothes they were wearing were relatively heavy and thick, which once again could've softened the impact even more and of course prevent lacerations.

They were also lucky that none of them fell on their heads; only Slavata of Chlum was injured but that was probably because he hit his head on the window frame or on a rock that was on the slope while he was rolling down.

Sources:

MORTIMER, Geoff; The Origins of the Thirty Years War and the Revolt in Bohemia, 1618, 2015

PETRÁŇ, Josef; Staroměstská exekuce, 1988