It's because the base of that model is really messed up. Zoom in really close on the bottom of that ring and you'll see.
There's tons of imperfections there that make the bottom sliced layer very erratic. What I typically do with models like this is move them down ever so slightly below the build table so that the rough bottom surface is now below the slice plane. The slicer starts slicing upwards from Z=0 so anything below the virtual build table won't be printed. Double click on the part and lower the Z-offset by ~0.25mm and it should solve the problem (see screenshot).
This is just a typical problem of poorly formed STL files. Hope that helps!