Why it happens
When a stew or sauce catches, the food in direct contact with the hot base of the pan dries out and burns. Burning breaks food down into bitter, smoky compounds. Stirring scrapes those burnt bits up and mixes them through, which spreads the taste. Thick, starchy or sugary sauces burn most easily, because they don't move around freely and the sugars scorch. If you catch it early, the rest of the dish often tastes fine.