In my opinion, the birds aren't behaving in a way consistent with toxic fumes or electrocution, not that I've seen how birds behave when they do. They don't look like their falling, they look like they are flying. They look lucid/alert enough to detect that something is wrong as the first birds hit the ground and take corrective action.
I suppose toxic fumes or electrocution could have affected the front birds causing them to fall and the birds behind them followed after trying to keep the flock together, trying to stick with the 'murmuration' (not sure what the term would be for these guys)