Question

Why the velocity is negative

Answer 1

Observe that the object below moves in the negativedirection with a changing velocity. An object which moves in the negative direction has a negative velocity. If the object is speeding up then its acceleration vector is directed in the same direction as its motion (in this case, a negative acceleration).

Answer 2

Negative velocity just means velocity in the opposite direction than what would be positive. The velocity is a 3-dimension vector, there is no such thing as a positive or negative 3D vector.