If you watch the landing footage of the Orbcomm OG2 mission, where the first stage of the F9 1.1 Full Thrust lands, just after it touches down, shuts down the engine, a triple boom is heard. 

Most videos of people watching it, seem to suggest they thought it exploded, but with no fire/explosion visible they get over that fear quickly.

Even Elon Musk [initially thought the booms were an explosion](https://youtu.be/LgNekVW0pps?t=4m45s). 

If you look at the various vudeos, it really looks that way, due to distance and speed of sound hitting the timing just right. 

In this [video](https://youtu.be/FdYGu9bTLhQ) the boom arrives before the landing, so still up high, less to possibly reflect off. 
But why a triple boom? Shuttle had a double boom, one from the leading edge of the wings, the second from the top of the tail.