She was stopped because she had a bottle of water and a sippy cup with water in it. According to their incident report, she flashed her secret service credentials and tried to bypass security. They then directed her back out to dispose of the water. Notice how she stops as soon as she clears the last guard and does something. It must be then that she dumps the contents of the sippy cup on the ground. It certainly didn't look like she was trying to drink it. That is when the blue shirted guard gets up and starts fussing at her. Eventually she is made to clean up the mess she made. So you can see where she was when she spilled the water.
“That is when the blue shirted guard gets up and starts fussing at her. Eventually she is made to clean up the mess she made. So you can see where she was when she spilled the water.”
She spilled it on a busy walkway so she should be responsible for cleaning it up.