Why random event follow a specific pattern?

I have create a grid layout. It displays 100 balls. Initially, all the balls will be in a random position. For every 25 ms a ball will move some unit (this is common for all the balls) in a random direction. You can see this in action in below image

Even though the direction of the balls are random, after some time all the balls are moving to a bottom right corner. This behavior is repeating ever-time. You can see this in the below image.

• Why random event follow a specific pattern?
• Are random numbers truly random?
• Is there is a problem with c# random number generator?
• Is there is any mathematical explanation for this?

C# Code

Random random = new Random();
var direction = random.NextDouble() * 360;
var ballTranslate = child.RenderTransform.TransformPoint(new Point(0, 0));
var x = ballTranslate.X;
var y = ballTranslate.Y;
var x1 = x + (parm.Speed * Math.Cos(direction));
while (x1 < 0 || x1 > (parm.CellSize * parm.NoOfSplit))
{
    direction = random.NextDouble() * 360;
    x1 = x + (parm.Speed * Math.Cos(direction));
}

var y1 = y + (parm.Speed * Math.Sin(direction));
while (y1 < 0 || y1 > (parm.CellSize * parm.NoOfSplit))
{
    direction = random.NextDouble() * 360;
    y1 = y + (parm.Speed * Math.Sin(direction));
}

TranslateTransform myTranslate = new TranslateTransform();
myTranslate.X = x1;
myTranslate.Y = y1;
child.RenderTransform = myTranslate;

Full Code

https://github.com/Vijay-Nirmal/ProjectChaos