If I had any good sense I would leave this one to Georg, but as I don't...

Firstly, I cannot promise that the brass was entirely innocent, but I am sceptical, to put it mildly. Undisturbed brass is not a very volatile substance. Zinc compounds do not typically have such catalytic oxidative  effects, and though copper often does, I cannot see it getting into the PE to any extent.Can't be sure though; does the brass get cleaned often, and if so, then with what?

Next, remember that there always have been various grades of PE with varying degrees of chain length, cross-linking and branching, which affect their nature, and their modes of perishing. Nowadays the sophistication and control of polymerisation are considerable, and one gets no end of grades with very impressive attributes, but especially the earlier grades had tendencies to crack, flake, go brittle, and generally to degrade in either gratifying or dismaying ways, depending on where and how they were used. I for example have seen PE behave in much the way you describe when it was exposed to sunlight for some months.

I am afraid that, not knowing which solvents were present, nor what grade of PE, nor the temperature and levels of sunlight etc, I cannot give you any better than handwaving. It is hard to put a finger on anything if I cannot inspect the corpse in person. Better luck with our other correspondents!

I'd rather leave this to Georg, too; but is the bag definitely PE? Some supermarkets have experimented with bio-degradeable aternatives in recent years. I don't know if this extends as far as their free carrier bags.

I think more or less the same

as Jon and Pete on that "PE" and brass. Instead of looking for "biodegradable"

You should look for "degradable". There is degradable PE since some years.

Sometimes called oxodegradable. The sellers promise embrittlement

and cleavage into biodegradable molecules within some months to some

years depending on conditions.

One hint: the laquer inside the mahogany cabinet might play some

role. Eg air drying laquers can split off peroxides even after decades!

I have seen a PTFE sleeve of a ball valve in contact with styrene

and some oxygen (pipe system not in use for about 2 years)

which was corroded having holes  like a cheese!**

Georg

To prove this, You could put a identical (!) bag into the cabinet

without the microscope.

** For those not familiar with PTFE: You can boil PTFE in 

aqua regia without any attac!.

The reaction you described sounds like it could be caused by oxidation. However, the rate of oxidation in PE is likely to be very slow without the presence of a stronger oxidizing agent than air.

My initial hunch was that the brass acted as an oxidizing agent, but metallic zinc and copper are reducing agents, so these are not likely the culprits.

Brass does not oxidize easily because a layer of ZnO and CuO already exist on the outside of the material. I guess it's possible that some of these oxides were reduced back to their metallic form in the presence of the plastic. That could cause the plastic to break down into monomer.

Of course, it could also be a physical change. It sounds like the plastic was stored under pressure. Extreme temperatures and pressure could cause a gradual phase change in the material over time. That could easily cause the plastic to become brittle.

Further suggestions:

What did the bag contain before?

The terpenes from the wood box? (terpene resin is polyethylene - I think.)

(Anaerobic) bacterial breakdown?

Evaporation of plasticizer(s)?

BTW I have also noticed the stiffening and resulting fragility of HDPE bags that have been in cardboard boxes. (5-10 years)