[two89w]

my 347 has 600mile on it and was on the dyno and it started to fume out of the rocker covers so they pulled the plugs out and 7 plugs were perfect but number 4 plug was black and had iron filings on it ,so the engine is out and the heads are off.number 4 piston has detonated on the edge of the fly cuts down to the first ring,and all other pistons are ok .i havent seen it yet but i will on monday,can some1 please explain how detonation happens?? and why only on 1 piston??does it have have something to do with the weber set up on that particular cylinder? and why not any other cylinder??