Birds do not perceive windows as a barrier. They see reflections in glass as open space and fly into it full-speed.
Male birds defending territories during mating season. They perceive their own reflection in glass as a competing male, and attack it repeatedly in an attempt to drive the intruder off. Robins and Flickers commonly exhibit this kind of behavior.
Disorientation due to disease and intoxication from eating fermented berries are less common causes of windows strikes.