1)Incorrect timing..Too much advance
2)a broken advance spring in the distributor base, a faulty vacuum advance mechanism
3) inferior fuel
4) Lean mixture
5) Overheating engine
6) Wrong temperature range spark plugs
7) Carbon build up in cylinder increasing compression ration
The proper term for this is "detonation" and is very harmful to your pistons... imagine hitting them with a sledge-hammer...
"Knocking (also called knock, detonation or spark knock, pinking in UK English or pinging in US English) in spark-ignition internal combustion engines occurs when combustion of the air/fuel mixture in the cylinder starts off correctly in response to ignition by the spark plug, but one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front. The fuel-air charge is meant to be ignited by the spark plug only, and at a precise time in the piston's stroke cycle. The peak of the combustion process no longer occurs at the optimum moment for the four-stroke cycle. The shock wave creates the characteristic metallic "pinging" sound, and cylinder pressure increases catastrophically. Engine knocking can range from inconsequential to completely destructive. It should not be confused with pre-ignition (or preignition), as they are two separate events."