空気圧！　ドーム球場などで経験する、内と外との気圧差による突風！！

…はともかくとして、こういうやり取りが無かったら、突然窓全開のマミさんの部屋にまどかは一人置き去りにされて怖いだろうなぁ…と思ったので

具体的には２コマ目を飛ばすとわかりやすいです

Air pressure! You experience this at places like baseball stadiums, where the wind gusts because of the internal/external air pressure differential!!

.... Well, that aside, if this sort of exchange hadn't happened, then Madoka would've been scared suddenly being left alone in Mami-san's room with all the windows open...or so I figured...

Specifically, it's easier to understand if you skip panel 2.