Any remote assessments of this situation are somewhat speculative; but I'll add this: I've never encountered a DO-crash due to the die-off of duckweed - whether the die-off occurred quickly or slowly (as with fluridone treatments).
Unless the surface-layer of duckweed was really dense, it may not represent enough biomass to cause a DO crash (on its own) - unlike a dense population of coontail or Najas, or a thick mat of algae, which often dies/decomposes very quickly when treated - and sometimes prompting a spike in BOD due to microbial decomposition.
BGK, I'd probably place a bet on your initial assessment: That is, running the aerator 24/7 increased the pond-water's temperature. Warm water inherently retains less dissolved oxygen than cooler water. So, if the 24/7 aeration dramatically increased the water-temp, the intended benefits of this action were possibly negated.
Just my speculative opinion.
