Answer: I would guess that over the course of a day and a half, evaporation caused the fluid in the straw to become less dense. Diet soda is less dense than water, and as the water evaporated out of the straw, leaving more concentrated diet soda behind, it would become even less dense than usual. The fluid in the cup would have experienced somewhat less evaporation because it's not directly exposed to air. As a result the comparatively more dense fluid in the cup would push the less dense column of fluid up the straw. Once you take a sip, you have removed the lower density fluid, and the level goes back to normal. BTW, the situation would have been reversed if you'd had regular (non-diet) soda in your cup because sugary soda syrup is denser than water.
BS Physics from the University of Maryland
Former Applications Physicist at the Super Conducting Super Collider (SSC)
Lisa from Michigan