Artist's commentary
MadoMagi 4-Panel [Has Spoilers] #07
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.