I'm guessing she was stocking the cooler from behind and he reached in and touched her? Maybe she's holding the last bottle of his favorite drink to tease him. I dunno either.
She was probably restocking the refrigerators from the back, when he reached in and accidentally grabbed something soft, making her jump and hit the racks as seen in panel 3.
But for fresh milk this expr date sometimes dont work as it is because the distribution process and the store's refrigation lack the cold needed to preserve fresh milk thus even before the expr date the package already swollen indicating the milk gone bad
Over here the milk isn't cooled at all. (see pasteurization)