DIAGNOSIS You're likely not using a thick enough liquid base or are applying too much heat after pressurization.
FIX Use a liquid that isn't just water. Broth, tomato sauce, or a sauce with some fat or starch helps create a barrier. More crucially, the moment the pressure regulator starts rocking steadily, immediately reduce the heat to the lowest possible setting that maintains a gentle, occasional rock. A high simmer, not a boil.
IF THAT DIDN'T WORK You may be not deglazing properly. If you sautéed onions or meat first, pour in your liquid and use a wooden spoon to scrape up all the browned bits (fond) from the bottom before locking the lid. Any stuck-on bits will burn under pressure.
HEADS UP Never use the pressure cooker to thicken sauces with flour or cornstarch directly in the pot under pressure. It will sink and burn. Always thicken after cooking, with the lid off.