Standpipe on the steam boiler return piping?

I can't say I've seen in this in the past, and I certainly can't explain it, but recently I went to a call where the complaint was water "erupting" out of a 1.5" pipe near the boiler. When I arrived, the boiler was already cold and turned off from the day before. The standpipe is installed on the wet return line near the boiler, and is piped up about 5-6' above the return line. Next to this pipe is a pipe that is tapped off the steam header, and directly piped into the return as well (no trap, valve, vent etc. in this line) The return line is then piped behind the boiler, and tees in at the Hartford Loop/equalizer. I feel like the piping is correct as far as dimensions/instructions go.

We started the system, and all was good until the steam header started getting hot. Then you could hear gurgling and some slight banging until that pipe started erupting. It took 10-15 minutes? At this same time, the water line was noted to be lower than normal, but still in the glass. (the operating water level is found to be lower than the tag for "normal" water level as well) and of course the boiler shuts off to take on more water.

Anyway, we did some routine things as it was our first time there. Cleaned the pig tails for both controls, flushed out the bottom of the boiler, and cleaned the LWCO/feeder probe. We let the boiler run, and it never erupted again once hot. The numerous air vents were all working. The water level was also found to be staying a little higher now once warmed up, but still below the normal line. I think this is because the Hartford Loop is piped in a little lower than recommended?

I realize this got long winded now- what is the reason for this standpipe, and what is the reason for the steam line piped down into the return line from the steam header? Also, if the Hartford Loop is piped a little short, can the steam equalizer allow steam down into the return, and up the standpipe?

Maybe if I understand this, I can figure what caused this all of a sudden. I'm told this is a 3rd year install, and this problem is new. I'm not sure I believe it's never happened before, but what do I know.

Thanks.