I have noticed from time to time when working in the Windows Forms designer that clicking back and forth between the designer pane and the properties grid can cause Visual Studio to think you double-clicked on a design item, and therefore create an event handler and switch to the code view. It is certainly annoying. I have recently found a way to prevent this.
When working in the property grid, press Enter after inputting or changing a value. Now when you click back to the designer, Visual Studio won't go all goofy on you.
Hope this helps