Question

in which case velocity becomes negative

Answer 1

negative velocity means the velocity in the negative direction.

Answer 2

Velocity can be negative, because it tells the direction as well as the amount of speed.
If the particle possess positive velocity it simply means that particle is moving in the direction which has been given tag of positive direction in some frame of reference and exactly opposite in the case when particle posses negative velocity. Positive and negative directions purely depends on frame of reference.