Why does trynd have a bag over her head.

"Her"

...

You don't need to know who your executioner will be. All you know is that you fear him or her (I guess?), and that his anonymousity could not help you in the slightest bit. Either that or he has just survived someone trying to murder him through choking with a sack. With all those chains you'd guess it wasn't the first time there was an attempt to end his life. It does not bother him anymore. Just let them try, and let their failed attempts pile on his body as warnings for those that are still to come.

Or... well... maybe because Tryndamere is a simple champion to who making creative skins is hard, and they chose to opt for more interesting, less common acessories (no other champion has a bag on his head as far as I know)