Collider triggers firing on launch - unity3d

Question

I have a "zone", with four borders; north, east, south and west. On these borders, I have 2d colliders, like this :
So, on a zone, I would have four instances of this, for each border. In the script for GridBorderController, I have this :
void OnTriggerEnter2D(Collider2D other)
{
if (other.gameObject.tag == Const.TAG_PLAYER)
{
print("enter " + gridBorderSide.ToString());
/*var pos = new Vector3(20, 20, 0);
var zoneInstance = Instantiate(this, pos, Quaternion.identity, this.transform);
var tileMaker = zoneInstance.transform.GetChild(0);
var terrainDrawer = tileMaker.GetComponent<TerrainDrawer>();
print(terrainDrawer.sprites.Length);*/
}
}
When I launch the game, I create an instance of this zone, and this is what I get in my console :
This is how I start the controller :
void Start()
{
boxCollider2D = GetComponent<BoxCollider2D>();
terrainDrawer = GetComponentInParent<TerrainDrawer>();
collider = GetComponent<Collider2D>();
collider.enabled = false;
var gridRadius = terrainDrawer.gridRadius;
float borderOffset = borderWidth / 2;
switch (gridBorderSide)
{
case GridBorderSide.North:
case GridBorderSide.South:
{
boxCollider2D.size = new Vector2(gridRadius/2 , borderWidth);
if(gridBorderSide == GridBorderSide.North)
{
boxCollider2D.transform.position += new Vector3(0, (gridRadius / 4) - (borderOffset));
}
else
{
boxCollider2D.transform.position -= new Vector3(0, (gridRadius / 4) - (borderOffset));
}
break;
}
case GridBorderSide.East:
case GridBorderSide.West:
{
boxCollider2D.size = new Vector2(borderWidth, gridRadius / 2);
if (gridBorderSide == GridBorderSide.East)
{
boxCollider2D.transform.position += new Vector3((gridRadius / 4) - (borderOffset), 0 );
}
else
{
boxCollider2D.transform.position -= new Vector3((gridRadius / 4) - (borderOffset), 0);
}
break;
}
}
collider.enabled = true;
}
When I walk over the collider with my player, it does fire as well, so I know that part works, but can anyone explain the me why the triggers fire immediately on launch?