Platy spawn question, in the wild

I keep my platies in a fresh water aquarium. I know they are supposed to like a little salt. Since this is full planted community tank (heavy on platies) I only end up giving about a tablespoon of aquarium salt to the 20 gallons about once every 1.5 months. I figure the water changes has the salt level about to nothing by the time I add more.

My observation is that once i add the salt after the last water change, the next day my platies are FAT. I have 3 females that look like they are about to burst with fry.

I'm wondering if there are certain times when these are more likely to breed? I also wonder if it could just be the water change itself. I've had corys lay eggs and breed before and that was usually after a large water change where the water that was added was cooler than the tank. I also waited for the barometer to change with incoming thunderstorms before those water changes.

Any insight would be great, more for curiousity than anything.