Melted white wire terminals?
Looks like they got loose. If the connection is loose it will arc and create a lot of heat if something that draws a lot of current is plugged in. 

It also looks like they got wet. There is rust and the copper wire has a patina. That would not help. If the outlets are below a water source that leaked, that could have contributed. Or maybe someone sprayed water into the outlets. 

I guess they're making receptacles even more crappy these days, I'm used to brass screws which would never rust (and are more electrically conductive than mild steel). The old bakelite (thermoset) plastic receptacles also would not melt. 

Incidentally, this is why connections are made in a fire-resistant box. There are also initiatives to incorporate mandatory arc detection into fuse panels. The arcing has a characteristic signature that can be used to shut down the circuit. Arcing still (rarely, fortunately) causes house fires, but it was worse when they were installing aluminum wire.