Why jQuery change my headers height ?

Why jQuery change my headers height ?

Hello

I'm using this website structure :  http://tympanus.net/Freebies/Cardio/

I use PHP to validate the contact form.
When this contact form is OK, the contact form disappear but the headers height change about 70000px.
Firefox says https://code.jquery.com/jquery-1.11.1.js line 4305 and 5211 change the header properties.

Do you know why?

Just tell me what isn't clear for you.

Antoine