This happened to me yesterday. I had numerous ideas before I made it home. I'm posting the clip so you can see if you can guess what it was (solution at the end) and so people can hear and see what happens just in case it's of use or ever happens to you.
The sounds heard are commonly telltale signs of a fuel delivery issue. The "waaa" sound is usually a lean condition. In addition to what's seen in the video, check things like petcock operation, float height, fuel line routing, carburetor jetting/adjustments, airbox/filter condition, vacuum hoses and connections. Four-strokes don't suffer them much, but the first scene is somewhat similar to a soft seize. Sometimes the engine just stops, other times it dies slowly. The engine usually starts after being off for a short period of time. These are caused by the piston expanding more than the bore and seizing up. Usually from improper jetting (lean) because of the heat the piston is subjected to. Some engines will do this quickly if WOT is used before the rest of the engine gets up to operating temp. Scuffing or scarring of the piston and cylinder bore at 4 corners is a common result of a soft seize.
Keep those ideas in mind and if your scooter ever does something similar, maybe it'll help you diagnose and fix it a little easier.