I just had the same thing happen. In my case it was wort stratification. I had topped off with clean cool water to hit my intended post-boil volume, thought I had mixed it in well, but after the yeast got active and mixed everything up even better then my gravity increased quite a bit.
If that's not it then I'm not sure why. The Tilt is a bit fussy, lots of variables. Carbonation, yeast activity, hops, bumping into the sides of the fermenter, all can have impacts on how it reads. Periodically I will rock the fermenter a little bit to see how the reading changes. Often it changes by several points when I do this, and I usually believe the reading after rocking more than the reading before rocking.