You may never know what killed your fish, but if the plants were typical aquarium plastic plants that had been thoroughly rinsed before use then I don't think they would be the cause of the issue. If they were plastic plants for some other purpose you might have something on or in them that could potentially cause an issue, ie some paint or colorant that is not stable or poisonous in an aquatic environment.
If the tank was established and you added the plants you might have disturbed something in the substrate that elevated, released, either the Ammonia, or Nitrite to a toxic levels for some of the fish. The disruption might even released methane from the substrate, that might have displaced enough dissolved O2 to cause an issue. All of this is just a guess.
Did you record the water parameters at the time, ie the Ammonium, Nitrite, and Nitrate values. Also how big is the tank and how many and how large are the fish.