Well like (I think) was already mentioned - the first form you create is called "the main parent form" and all subsequent forms are "child popup forms". When a child form is made visible, the main parent form is disabled for user input. When the child form is hidden (or destroyed) the parent form is re-enabled. This is all by design and it was meant to help control serialization in the script.

It doesn't "have" to be this way but it was just a design decision that was made early on.

-Shawn