This is a video of Lehigh University professor Jerome Licini discussing, at length (some might even be inclined to argue at too much length, but those people wouldn't know science if it took a swing at them with soap-on-a-rope), why empty or near empty shampoo bottles are so easy to knock over. The simple answer? A nearly empty bottle has significantly less mass than a full bottle, and the center of that mass has returned to around the middle height of the bottle since the majority of shampoo has been used (as it was being used the center of mass traveled lower and lower along with the shampoo level, keeping it relatively sturdy). An even simpler answer? All the slippery soap suds in the shower and my penis swinging around like an elephant's trunk.
Keep going for the full video and learn all you need to know to call yourself an empty shower bottle physics expert.
Thanks to Mister Wrong, who, wow, well thanks for telling me before trying to set you up with one of my friends for a virtual Animal Crossing date.