Boiler flooded. Why?

Gobbaloon

Hi, I have a Burnham V904 (originally oil but converted to gas in 2009). One pipe steam system. 8 units.
DHW is from an indirect storage tank.
Three days ago there was no heat/hot water. Turned out the boiler was flooded.
I drained about 35 gallons out of the boiler. The boiler came on again about the time the water level appeared at the top of the sight glass but I kept going till level was where the McDonnel-Miller water feeder/LWCO wanted it. Problem solved.
Everything has worked fine since then but I want to know what caused this to happen.
But there's something else. There is a water meter on the system which does not show that 35 gallons fed into the system. (I track it weekly.) Is it common for these water meters to fail or malfunction? Or is water possibly getting into system from the coil in the storage tank? But, if so, then why is system working well again?
I'd be happy to hear diagnoses from experts here.
The main question is : What caused the high level of water in the system?
Another thing I'm curious to know is: What is it that prevents the boiler from firing when water is high?
A pic or two attached
Thanks for any light you can shed.

Gobbaloon

Pics here.

nicholas bonham-carter

If the water level is too high, then it’s standing pressure will trip the pressuretrol, and prevent the boiler from firing.
Keep an eye on it, as you may have a leak into the boiler, from the hot water system.—NBC

JUGHNE

If you are confident that your LWCO's will shut the fire down as it should, then you could shut off the cold water supply valve to the boiler for a few days. If it over fills then your coil would be the suspect. You can do the test the other way also shutting off the coil for as long as you can do without hot water.

Just be sure to test your LWCO to shut down the burner.