Does your spawner object have a collider on it? It could be colliding with the grenades when they spawn and pushing the spawner down each time. I had a problem like that with a past project.
Based on how you described it I don't think that's the problem but its something to double check.
You might be able to add a rigid body to it if it doesn't have one, and go to constraints in the inspector window and lock the y position.
If that doesnt work, a work around would be to make it set its y position relative to the capsule everytime it fires though that certainly wouldn't be the most efficient thing.
Sorry I don't have a more concrete answer but I can't tell what would be causing it unless its a collision, FSM, or script attached to it or a relating object.