My experience with connectors and heat is that they get oxidized a bit and start micro arcing (is that a term?). That generates heat and more oxidation as well as carbon which increases the resistance. It's not that the material's resistance increases, just that the connection gets worse due to oxidation and carbon. And gets hotter.
I've removed a 20A 2 pole breaker that had a loose wire. One side of the breaker was melted away and it never kicked because the current was never over it's rating. (Cutler Hammer Square D clone, back in the late 70s' early 80's).