How come bosses die when above 0% ?

[Danzoo]

It's room for scripting, the way the engine works is if someone hits 0hp they die and can't do anything else. Some bosses reach a certain amount of health and then become non-interactive so they can do their scripts. If you don't have this large amount of health, Overkill damage can bug it out.
Also, server lag is another reason, so is calibration between the UI, Client and server. It's a whole number of reasons.

[Eurytos]

were the videos sped up?