I sometimes feel like there is a hidden "bad luck" trait...
I have had a ton of issues with fires with my current Sim household. When they had a toddler...she constantly was putting her wabbit tablet down too close to the fireplace. I think we went through...
My theory is that these unfortunate things are built into the game, and I do think some sims are genuinely unlucky. However, there's definitely a way to make it more noticeable.