If you have just a few fish die it may not be a problem. It's not unusual for a pond to lose a fish or two now and then especially older fish.

On the other hand something may be stressing them as some anoxic water coming up when the pond really starts mixing after colder temps. Or it may be your pond temps drop too rapidly which stresses them.

Do you destratify in the summer?

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