An iPhone's CMOS camera has no shutter, and reads the pixel values off in rows rather than all at once. In this photo the propeller spins about 5 times while the photo is taken, producing a strange effect.
Note from the photographer: "We take a lot of aviation type pictures during our normal day, from crappy workmanship we find in planes to cool flying shoots. This prop shot was way different that any other prop we have shot. If you watch the prop through the camera the blades spun down and came off the prop at the 6 oclock postion, then looked like they bounced off the ramp and reattached at 9 oclock. Very weird, Hope next week we can recreate with a better camera and some video. Aircraft was a Yak TD with a M-14 engine idleing at 1000 RPM, Yes we had someone in the seat and brakes were set. Should of had chocks too. As for Photo shop, if I had those skills I would not be working on communist iron."