I logged in this morning and was awarded this achievement and the box of helms. I am assuming this is bugged, and just thought I’d post here in case you guys weren’t aware. 
The Big Boom in the Big City Achievement
The bug report is here: https://forum-en.gw2archive.eu/forum/support/bugs/Dragon-Helmets-Bug-LA-Overflows
I support Portable Doors: Player Housing anywhere!
~ Whips ~ City Minigames ~ City Jumping Puzzles ~
~ Whips ~ City Minigames ~ City Jumping Puzzles ~