A more scientific TFPer will probably be able to clarify or disprove this, but I think that the alcohol in the Corona allows it to reach sub-freezing temperatures (i.e. below 32F). The water molocules in the Corona is seperated buy the alcohol and can't get together to form ice. The lime (or any solid object for that matter) breaks the surface tension enough to allow some of the water molecules to latch onto each other and the whole thing freezes.
That's just my best scientific guess. You should try putting salt in one. It errupts in fizz.
