I've wondered this too. I blamed the ammo. I switched to a different brand for target practice (it was just the cheapest at that particular store) and the same thing happened with my 1911. I just figured since it never happened before, and it was uniform, it must be the ammo.
Of course the casing gets hot- you just had a fire/explosion inside of it- the heat is what causes the rapid expansion of gasses. Wait until you get a hot casing drop down the neck of your T shirt (Well, looky there- he CAN dance!)
A mild powder charge may not fully an immediately seal the brass case against the walls of the chamber (as was already said up there). Some of the carbon from the powder burning is pushed between the casing and the chamber wall, leaving little sooty marks on the case. Normal.