1 Elementary, my dear Watson.

Super Sonico is five-foot-two. A tennis net is three feet high. Given the ankle-high camera position, she must be very close to the net for the entire net to be visible in her... um... "A-frame"... when she's bending at the knees like that.

So she must be playing the net in a doubles match. Her hapless partner just returned a shot inaccurately, right at the back of Sonico's head. Her partner yelled "abunai!" causing Sonico to spin, sidestep, and duck slightly, flaring her skirt out and whipping her hair around.

Sonico is not hitting the ball; she's avoiding being hit by it.

2 Actually, I think it's easier to explain by saying she was trying a backhand and she missed.

3 Hm.  Her racket arm is in a strange position for that.  

I'm trying to recall if she was clumsy enough to pull that off (spin 180 on a missed backhand, and swing early enough to complete the spin just as the ball arrives).  I guess I'll have to watch the anime again.  Darn.