I would say shocking is highly unlikely. Shocking gear in a boat requires an investment of of several thousand dollars(I believe 9 or 10 thousand dolloars). The average Joe can't afford that nor would have any reason to invest in it. Along with that, it would take some gile to come in with a boat and trailer not knowing if you would show up.

I say someone is, or was fishing when you are gone, or some type of natural thing is taking place as in predation by large bass as previously proposed.

There is another possibility. In my pond once the bluegills get really large they are much harder to catch and frequent deep water close to the bottom. Are you trying deeper water or live bait?

If pigs could fly bacon would be harder to come by and there would be a lot of damaged trees.