Because he is uploading them to the board (instead of youtube) which isn’t designed to serve videos
At least not the huge files his phone creates . This one is 118mb for 44 sec which means that anything below a 24mb/s download speed it takes longer to download than the length of the video causing buffering/pause. Slower speeds get worse .....there are also server constraints that can come into play.
Better to select download than to try and watch them on the board (hit the 3 dots and select download)