SING then goes on to explode due to the high tmp value it inherited from the SNOW form. In other words, the way to make exploding SNOW (or LAVA or ICE) is set ctype to SING and tmp to something big (say, 5000).
The GRVT don't disappear becuase their life is set to 0. Snow #1 is not passive; put it in a box with any other element and it'll explode, just like Snow #2. Now, why it explodes is a different matter. It's because it's technically "frozen SING". SNOW stores in its ctype what the frozen form it is of. If this is SING, it's frozen SING, so when it reaches melting point (by, for example, coming into contact with warmer particles), it turns back into SING.