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Here's one way to see that the pattern holds regardless of the grid configuration.
Let's say an explosion takes place in cell c at time t = 5. That explosion would have wiped out c along with all of c's neighbors. Because we wiped out all of the neighbors, there's nothing left to stop the next bomb at c from going off four seconds later, at t = 9.
So more generally, once an explosion takes place in a cell, that cell is guaranteed to explode every 4 seconds from then on.
The first explosion takes place at t = 3. So that's why we repeat once we reach 3 seconds.