Copper sulfate is used to kill filamentous algae, which is the slimy green stuff you have growing around the edge of the pond in the second pic. The standard recommendation is, if a pond has a good deal of FA growing, not to treat the entire pond at once because what happened to your pond can be the result. Dying weeds take up oxygen; probably the pond had a lot of FA and the previous owner got a little slap-happy or impatient or just didn't adequately research the application of the chemical. It's a shame too - looks like the pond had some nice-sized bluegill in it.
