What reasons led to the amount of casualties in WWI?

then trench warfare was a new kind of war, same goes for the fighting with airplanes, it was the 1st time airplanes were used in war. new technology made trench war more deadly, they had quick firing artillery. Poison gas was used on each side. also, the brits invented the tank and it cleared paths in "No man's land" (the area between both sides, surrounded by trenches) U-boats, or submarines were equipped with guns and torpedoes too.