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You are quite correct. There should be no pressure -- and certainly no steam -- in the dry returns. This suggests, though, that there are two separate problems. One is that somehow steam is getting into that dry return. A connection which shouldn't be there, a failed open radiator trap, a pair of drips to a wet return which isn't actually wet (too high -- above the water lien), a blown water seal (too much boiler pressure)... several possibilities.

It is likely that the presence of steam closed the main vent on that dry return which allowed the pressure buildup-- but... is there one? Is it functioning?

Go back to basics and take a look at everything -- and think like steam (which should be in the mains and radiators and nowhere else) and air (which has to get out of the radiators and then out of the dry return) and water (which has to get out of the radiators, into the dry return, and then through a drip to the wet return or boiler.

What size boiler ? Is that a 3 inch riser or was it reduced? Was this boiler skimmed ,from the drop on the equilizer blocking the skim port i would think not . I see no provision where left to wand the boiler or drain all contents out of the boiler . Possible you have some carry over condensate from the boiler being possible dirty . Are you using a vapor stat as a pressure control or still the pa404 . As other may have asked is the a good low pressure gauge aside from the zero 30 gauge . I usually add nothing and just flush wand and skim boilers I ve installed , some times it takes 2 or 3 times to get all the crap out and at last I’ll add surge master when I’m sure it as clean as it can be just for the o2 and for slight corrosion protection and a great indicator of make up water when it ain’t green they have been adding make up . Follow what other have stated and if there using water seal type traps and graduated supply valve then it gotta be a vapor stat and and low pressure other wise you will be just chasing your tail like a puppy . Usually no part of a steam system I replace has any copper except the water line feeding it . Full port tees w bushing and ball valves for flushing and draining other wise it just glam .
Peace and good luck clammy