Wow... I used that too in 'The Lost Tribe'... That part is annoying for other players. But, you promised not to kill any turtles 'yet' So you insta-kill when you kill a turtle.

Edit: You die because of many many TNT-Crates beneath the bridge that are triggered when touched by a shell. The whole bridge explodes and you fall into a warp, which will bring you to the CTF bases, so you die.