It happens because in a busy airport when one aircraft lands and doesnt clear the runway on the turn off he expected and has to take the next one being on the runway longer than expected, the aircraft going to land behind has to go-around due to the tight spacing between them. Or, an aircraft that is suposed to take off has to abort the takeoff and therefore is still on the runway so an aircraft behind cannot land and has to go-around. It happens sometimes and nothing to worry about as pilots are fully prepared for this. The result, as you would have seen, is just to fly around for another normal approach and landing.