How the window was broken is unknown. The floor in question has been empty for some time. The outer pane seems to have been smashed, while the inner pane is still intact.
The upper floors of the iconic Tinbergen building on K.P. van der Mandeleplein have not been used for a year and a half, first because of a legionella contamination and then because of fire safety problems. The building is likely to be renovated before reopening. In 2018, the tender for that renovation failed.
Update Thursday 10am: The square was opened up again on Wednesday afternoon. However, there are still some glass fragments lying on the square, it is unclear whether this fell down from the fourteenth floor..