maybe the relay was on it's way out (if they do wear out that is) because usually a high resistance in a circuit (perished wires, dodgy connections etc) drawing a lot of current will generate a lot of heat, hence the meltage.

other than that i dunno, not unless your pump is trying to draw more current, but i think your fuse would blow first as that's what they're for? unless someone replaced it with a higher rating fuse or tin foil