That is awful! I would think that a chemical was on the new pump that caused this.

How did you eliminate electrocution as a cause? It would be my first guess.

Second guess, if the pond had a very, very high fish load would be low O2. I think this is less likely because it would have to be a really high fish load and the water in your area is cold which holds O2 better. It does fit with larger fish dying in a short time.

Another guess would be poor pH buffer and high fish load would cause CO2 to build and pH crash. Pretty unlikely though, normally a pH crash isn't that deadly so fast.

Chemical coming off the pump seems unlikely to me. Smaller fish should have been effective first and the chemical would still be in the water and kill all the fish. Plus what chemical could this be? I can't think of any a pump could generate that would act like this.

I read a case once of a pond heater shorting out and burning up. It was an indoor pond and the flumes filled the air in the basement and the fish all died (if I remember correctly). Chemical poisoning is more likely there because once in the air chemicals could then get into the water, but electrocution was still a better guess imo. Chemicals entering the water directly from the pump is difficult (impossible) because the temp can't get very high. Plastic will melt at a pretty low temp but chemicals from plastic and oil need really high temps like a fire or arching. That's hard to accomplish in water.

Chemicals in general, from any source, would be really difficult to see how it could kill all the large fish in an extremely short period and leave small fish alive. Over a long period, weeks, months, I could see.

Electrocution is the only reasonable guess I can come up with.

If you wanted to reduce risk of electrocution in the future the pond water can be grounded and/or the pump placed on a GFI circuit. GFI circuits don't last forever so if you're keeping a large fish load I would suggest 2 separate circuits, 2 separate pumps. Or one water pump and one air pump.