EEEK! I see you folks are updating things, so I'm hoping this works itself out. When I add a new task, it reorders the whole list! Yikes!
Which view are you in when this happens, a Project, the Today view? Which browser is this with?
Next 7 Days View
The only way to add a task on the 7 Days view is to use the Quick Add button at the top (+ button), and when you do this, it changes it's view to the project it was added too.
This is the intended behavior. We will consider changing it in future updates.