Newbie Question: Funkey Pattern

[Comreak]

I have a question about some program I made while playing with vb.net's drawing functions. Here's the code:

Public Class Form1
    Inherits System.Windows.Forms.Form
    Dim x As Long, y As Long, z As Long
    Dim black As Boolean, red As Boolean
#Region " Windows Form Designer generated code "

    Public Sub New()
        MyBase.New()

        'This call is required by the Windows Form Designer.
        InitializeComponent()

        'Add any initialization after the InitializeComponent() call

    End Sub

    'Form overrides dispose to clean up the component list.
    Protected Overloads Overrides Sub Dispose(ByVal disposing As Boolean)
        If disposing Then
            If Not (components Is Nothing) Then
                components.Dispose()
            End If
        End If
        MyBase.Dispose(disposing)
    End Sub

    'Required by the Windows Form Designer
    Private components As System.ComponentModel.IContainer

    'NOTE: The following procedure is required by the Windows Form Designer
    'It can be modified using the Windows Form Designer.  
    'Do not modify it using the code editor.
    Friend WithEvents PictureBox1 As System.Windows.Forms.PictureBox
    Friend WithEvents Button1 As System.Windows.Forms.Button
    <System.Diagnostics.DebuggerStepThrough()> Private Sub InitializeComponent()
        Me.PictureBox1 = New System.Windows.Forms.PictureBox()
        Me.Button1 = New System.Windows.Forms.Button()
        Me.SuspendLayout()
        '
        'PictureBox1
        '
        Me.PictureBox1.Location = New System.Drawing.Point(16, 8)
        Me.PictureBox1.Name = "PictureBox1"
        Me.PictureBox1.Size = New System.Drawing.Size(280, 280)
        Me.PictureBox1.TabIndex = 0
        Me.PictureBox1.TabStop = False
        '
        'Button1
        '
        Me.Button1.Location = New System.Drawing.Point(112, 296)
        Me.Button1.Name = "Button1"
        Me.Button1.TabIndex = 1
        Me.Button1.Text = "Button1"
        '
        'Form1
        '
        Me.AutoScaleBaseSize = New System.Drawing.Size(5, 13)
        Me.ClientSize = New System.Drawing.Size(312, 334)
        Me.Controls.AddRange(New System.Windows.Forms.Control() {Me.Button1, Me.PictureBox1})
        Me.Name = "Form1"
        Me.Text = "Form1"
        Me.ResumeLayout(False)

    End Sub

#End Region

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        'cycle 1
        For z = 0 To 280
            x = x + 1
            y = y - 1
            If black = True Then
                PictureBox1.CreateGraphics.DrawLine(System.Drawing.Pens.Black, x, 0, y, 280)
                black = False
            Else
                PictureBox1.CreateGraphics.DrawLine(System.Drawing.Pens.White, x, 0, y, 280)
                black = True
            End If
        Next

        x = 0
        y = 280
        black = True

        'cycle 2
        For z = 0 To 280
            x = x + 1
            y = y - 1
            If black = True Then
                PictureBox1.CreateGraphics.DrawLine(System.Drawing.Pens.Black, 0, x, 280, y)
                black = False
            Else
                PictureBox1.CreateGraphics.DrawLine(System.Drawing.Pens.White, 0, x, 280, y)
                black = True
            End If
        Next
        x = 0
        y = 280
        black = True

    End Sub

    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        black = True
        x = 0
        y = 280
    End Sub
End Class

I understand what I did here but I don't understand where the strange patterns came from. Run it and you'll see what I mean.
Anyone know why it does this?

Download the attached file if you don't feel like fooling around with the code.

[applescript]

I don't see what happened, but that pattern kicks ass.