A lot of people say that “UL listed” products are considered safe, but I haven’t yet figured out what that means.
It’s hard to understand how one of these things can cause a fire.
Here are some options: 1) it was left on the hottest setting and it is meant for cooking and not just warming, so it gets so hot it catches fire itself; 2) it is on a hot setting and the food in the pot burns a bit, enough to catch fire and send sparks up, which catch fire to other parts of the house; 3) things like oven mits or rags that are near or touching the warmer/hotplate get hot and catch fire, which spreads elsewhere; 4) there’s an electrical problem and the thing just catches fire for no apparent reason (even though it’s not on an extremely high setting for a long period.)