Any idea what would cause this? Found it smoldering today.

https://imgur.com/gallery/kY9DHZh

Loose wire. I would check all wiring connections at the plugs. They do loosen over time. Make sure they are tight.

Looks like aluminum wiring in the background. I'm guessing that outlet isn't rated for aluminum and you had a connection come loose. Either pigtail the aluminum properly or you need to get CO/ALR rated receptacles. Also, if that is the case (aluminum wiring), your problem could be house-wide. If so, you need to get it addressed sooner rather than later.

Correct on the aluminum. We have replaced a lot of of it but still have it in some of the house.