This probably happens when you type a space. Are you using a laptop? If you have your hand rested on the laptop while you type, you may click outside of the text area and that will cause the browser to assume you want to travel to the bottom of the page. It happens to me all of the time. You may be able to deactivate this setting, but you will have to look up the way to do it on your particular computer model (or browser).
If you are using a mouse, you may have to clean it to make sure it isn't clicking (by itself) outside of the text area. I hope this helps.