Theory 1: Biology, or 'the kick'. A person can't give an entire speech in a shout. It'd be too tiring. Perhaps, much like a runner in a race, they save a bit of energy for the home stretch; many coaches tell long distance runners to 'kick it out' or sprint when the finish line is in sight, no more reserve energy is required, you don't need to pace yourself anymore. (I've also heard the reverse arguement that if you pace yourself properly, you don't HAVE any reserve energy left for a sprint, you peter out simultaneously with crossing the finish line. But that doesn't fit my theory, so meh, lol.)
Theory 2: Relief. When I was in boot camp in the military, recruits were punished for misdeeds by getting 'bent' or 'smoked'. Which meant forced physical exercise for an extended period, during which we all had to count out loud the number of repetitions of the exercise. Often the instructors would tell you ahead of time a number of reps that would be done, say 200 for example. There was a phenomenon where all the recruits would begin counting louder at about 190, in relief that it was almost over. Sometimes the number would be extended then to 300 because the instructors noticed this too and got angry that we weren't that loud the whole time. Perhaps the speaker is nearly as relieved as the audience that it's almost over with.