"Non-combustible" or "self-extinguishing" might be true if you try to burn an individual piece of that plastic, but an entire mass of it can be a different story.
Agree. The "self-extinguishing" material rating is misunderstood by many and would appear so in this case in the decision to store probably what was UL-94 rated plastic material under the bridge.
The problematic part of that rating is that the self extinguishing test is in part a heat transfer issue.. A single specified piece of material suspended per the test may well receive the rating, but that same material in a confined area, with reflected heat, other ongoing ignition source, or otherwise different heat transfer/escape characteristics means it may burn rather violently.
Learned this in an evaluation of an issue in a fairly large digital system using an extensive amount of ABS UL94 rated plastics. The plastics first melt into a black tar like residue (plastics being a petroleum derivative) and then ignites and burns with the ferocity of a petroleum fire. The materials engineer in this investigation mentioned that the bromine and other compounds added to plastics for the flame retardant rating yields considerable chlorine gas when burned.