Why does water hammer occur randomly when turning on a tap?

[wolram]

Why does water hammer come and go, i can turn my hot tap on say ten times, and the water hammer will only occur six times.
http://en.wikipedia.org/wiki/Water_hammer

 

[chrisbaird]

It depends on how quickly you open and close valves. If you close a valve quickly, the water inside the plumbing still has velocity but no where to go, so you get a shock wave. If you close the valve slowly, the fluid in the plumbing has time to slow down. If you have dishwasher, washing machine, etc, drawing water then this effects the fluid dynamics in the plumbing.