Mephisto really explained it well. The building has many panels and in the early afternoon the reflections all meet up on the edge of the sidewalk. I also had on a circular polarizer which knocked it down another stop or so.
coming back to this, i now see how it was possible that the reflected light appears to be way stronger adn brighter than the sun light itself, as it seems to have overlapped from multiple windows, just like a concave mirror would bundle (if that's the right word) resp. concentrate the light. pretty fascinating stuff... could you post your camera settings? i'd love to know because i imagine you had to stop way down to get a decent exposure in this bright spot...