Does anyone understand the physics of why a ball swings like that?

[IUFG]

Have you never played football?

When you hit the ball sweetly the ball doesn't have a high spin rate.
Spin on the ball give it a predictable ball flight.

When it doesn't spin (or has a low spin rate) the ball flight is erratic - I assume wind resistance plays a part.

here you go...turbulence around the seams apparently...

http://news.bbc.co.uk/1/hi/magazine/5048238.stm