To my understanding, it was because Uschi felt it necessary to make her Mother laugh. She did this by squeezing her shoulders up to her ears (making her neck vanish), stretching the corners of her mouth to the bottom of her face and squinching her eyes together and sucking in breath while making the weez of an old man (see picture to help illustrate it). She also tends to open and shut her mouth like a fish gasping for air, while making a "na na na na na na" sound. During such performance, she has completely serious eyes. But once the face is done, laughs at herself as she realizes her comedic genius.
Uschi also loves to grab a hold of her dear mother's face and make alien or animalistic sounds while contorting her face in abnormially humours ways.
Apparently her little daughter has become a little comedian...and is quite aware of her whit!
