I think the low quality ovens etc are more likely to catch fire, too. All my households seemed to have a fire the first time someone cooked. I had my kid start a fire using that potion table thing, too. I did notice accidents tend to happen more if a sim is not in a good mood, or they are really tired. My sim who is level 10 in cooking and gourmet cooking, who always makes excellent quality meals, cooked something that was only "good" because she was uncomfortable from being so tired. So maybe making your sim really tired will be more likely to trigger a fire?