It seems that in Couter Strike:Global Offensive on de_nuke there are no stairs in the outside area (head right from terrorist base) to the lower bomb spot. This was the case in CS 1.6 and seems to be the case in VODs from tournaments. I assume it just is not there and another version is played in tournaments, right? Or is there a button or trick to open/reveal the stairs?
The version used by most tournaments is de_nuke_ve, which was remade by professional player Volcano. You can read more about the changes he made here. The main (relevant) difference is that he moved the stairs down to lower to a more accessible area outside. Note that this is unlike any previous version of de_nuke.
Valve's rationale behind the removal of this pathway was that the stairs were very underutilized anyway and that this thus streamlines the action, and while that was true removing it makes the terrorist options incredibly limited and allows the CT's to consolidate even more easily.
As of the patch on December 12, Valve has integrated most of the key aspects of de_nuke_ve into the standard version of de_nuke used by matchmaking.