There are a few possibilities:
This site about pet turtles mentions the possibility that turtles fighting over a mate (or actually mating) may end up on their backs.
When breeding season comes around, adult male turtles might start
fighting over the females. A stronger male might flip a weaker one
over. Male turtles might also harass female turtles, attempting to
breed. Females can become flipped over and injured. Male turtles can
also fall onto their backs after mating.
As Sue says in the comments (and backed up here) fluid in the lungs due to respiratory illness may cause them to tip if they're submerged. A flipped turtle could then be washed ashore and remain upside down.
This brings me to another possibility:
As waves come to shore, the water molecules in those waves are spinning in a circular motion. With large waves, this is often enough to flip things over (surfboards and people on surfboards are the most common examples).
The part of the object closest to the approaching wave gets lifted first and thrown over, if the wave is steep enough. Breaking waves are exactly this phenomenon--the top of the wave is steep enough that the circularly moving water gets "thrown" forward and over itself. Waves usually break near shore--hence the turtle on the beach.
Also, it's worth noting that there's no reason the turtle would only flip 180 degrees. In all likelihood, half the turtles landing on the beach are right side up, and they just walk (?) away.
In the interest of being exhaustive about the possibilities, I want to mention that there are people who may think it's funny to flip a turtle on its back. I hope that wasn't the case here, but worth mentioning.