It's is more of a problem with fructose in general. When your body absorbs sugar, it can do two things: It can process the sugar immediately for energy or it can store the sugar for later as glycogen (a sort of compressed version of sugar).
When cells are processing the sugar for energy there are a few checkpoints where the cell checks to see if it has enough energy. If there's plenty of energy, the cell stops the process. Fructose enters this process after the checkpoints.
Excess energy generally gets stored as fat. It's not quite as simple as this, but that's the basics on why fructose is a shady sugar.