The camera was probably just writing the data to the card. Especially if you were shooting RAW+L JPG (like I do sometimes) or if the flash was on (for whatever reason).
Basically it is just filling out writing to the queue. Mainly will happen when trying to take multiple pictures in a row.
One more thing that I forgot to mention was that for a few of the shots I couldn't get the full explosion just due to how close I was to them.
