It's a gauge for the quality of a video that doesn't require someone to have to sit through it to see if it's truly worthwhile. It also doesn't make much sense to track both likes and dislikes, but only show the value of one of the two.
I know some people who do YouTube as their career and there are people mass downvoting videos within seconds of them going live to affect their visibility.
Personally, I choose to hide both on most videos I make.
Why does the mass downvoting happen?
Seems like an automated attempt to harm a video from becoming widely seen.