Bubble Rain is an event that occurs in Bubble Witch 2 Saga. This reward is triggered when a player finishes the level with leftover moves. You will not receive this reward unless you complete the objective that is required to commence.
If you have left over moves, Bubble Rain will activate and score you additional points. Bubble Rain is when the bubbles fall of the top and come down and score you additional points. You will know when you receive Bubble Rain, when it shows Stella with her wand facing up and Big bold letters will read Bubble Rain. However, there are no voices, or any "strings attached" to make this thing occur (unless refer to method).
Difference between Bubble Witch Saga and Bubble Witch 2 SagaEdit
It does not say "Bubble Rain" at the end it just brings bubbles popping out of the couldren (up to a maximum of 9 bubbles even if the cauldron has more than 9 bubbles after clearing 9 out of the 17 ceiling bubbles), all those will score you additional points.