It was overcooked that's all. 20 minutes is a long time for something like salsa that is already cooked. Remember you are cooking whatever you are processing so 20 minutes would be a long time even for many raw vegetables. Try less time, 10 minutes for pint jars aught to be plenty imo. I actually don't even process salsa as my recipe has vinegar in it which makes it acidic enough not to spoil. I never process anything with vinegar in it like relish or pickles, chutney etc. for the same reason (there is no need and it overcooks things) I pour it boiling hot into hot sterile jars and seal. The jars always seal and I have never had it spoil. the secret to no spoilage is boiling hot and sterile jars (I bake my jars in the oven at 250 degrees to sterilize them and boil the lids separately. don't touch anything with your hands)
Keeps in a cool dark place at least a year. (but it never lasts that long
KO
Karen