Salt turned peas from creamy to crunchy
Frozen peas don't really need 'cooking' at all.
The smaller they are the less they need, too; so anything labelled 'garden peas' or 'petit pois' really all you should do is drop them into boiling water, stir &amp; give them maybe 1 minute maximum to heat. Drain &amp; serve immediately.
Don't wait for the water to return to the boil, assuming you have maybe twice the volume of water as peas, just keeping the heat under them for a minute will be sufficient.  

They will keep cooking as long as they remain hot, which is possibly why you thought adding the salt afterwards was the trigger. It wasn't, it was that they will simply keep cooking; so the trick is to drain them early rather than late.
I usually drop them as I'm serving the rest of the meal, so they're not waiting around at all, they're just the last thing onto the plate. If I'm adding to something like risotto, I will add right as I'm going to serve. Drop, stir, serve. They'll finish cooking on the way to the table.

If you leave them too long - 2 or 3 minutes is all you've got, maximum - then they will just get harder &amp; harder over time. There's no rescue at this point, they're ruined. Switching off the gas isn't sufficient to stop them over-cooking, you need to get them out of the hot water straight away.

If you want them salted, salt the water first; though I never salt peas, they don't really need it. I do salt almost all other veg, just not peas.

The same, incidentally, applies to sweet corn. On the cob, 6 mins max; as individual kernels, drop them in boiling water, stir, done.