CBS news is still saying she was sitting on the window ledge and she slipped out of the grandfather's grasp.
Both sets of grandparents were on a trip with the parents & children. It could be the first time the grandfather got to spend some real time with the toddler. He may not have had a real clue how they can turn into all squirmy arms & legs, all of a sudden, while holding them.

And that a little kid can pitch themselves with more force than you'd think their 25 lbs could do.

Maybe she & the grandfather went for a little walk. They ended up along the wall of windows. But, at 18 months old, she's not really comprehending what's out there & losing interest. Someone else had possibly left the window open after they just had to take the perfect vacation photo. They didn't want to do it through a pane of glass. So they opened the window & unfortunately left it open. The grandfather heard noises out the open window and thought his little granddaughter might see or hear something interesting through the open window so her perched her on it.
As Mackenzie said above about the Bellagio fountain, he wasn't thinking danger. His focus was on keeping her occupied & interested in something. Maybe she did hear something and she lunged forward with all her weight, to look, and he didn't have a good grasp on her.