Because Internet Explorer is a horrible browser and you need to immediately switch to Firefox.

When you use the mini-calendar along the right side, especially on the home page, or put your mouse over other spots on the page where little informational boxes pop up, form fields (like Login boxes and dropdown boxes) disappear until you're done. Why? Because Internet Explorer, among many hundreds to thousands of nasty bugs and problems, cannot display these things properly.

You need to immediately switch to Firefox. It is free, easy, and it imports all your Favorites and passwords and stuff. Most importantly, it is not the most dangerous piece of software on the planet, responsible for almost all the viruses everybody ever gets. What is? Internet Explorer.