Why the shudder?

I was experimenting with my camera at badminton tonight. The question in the title refers to the apparent 6 images that make up the moving racket.

My theory -

I think it is due to the lighting running at 50Hz supply, but...........

If that were the case 6 times 1/50th of a second is slightly less than a 1/10th of a second. However, the recorded shutter speed was 1/15th.

Ever wondered how accurate shutter timings were? Then, of course, I could be barking up the wrong tree.