It seems a bit odd that a migrant can have the mental fortitude to hold it altogether while a fort crumbles around him, survive a dangerous trek through wilderness in the hope of reaching a new home to build a new life, and then go stark raving mad the instant they set foot in their new home. Perhaps its the last straw when they recognise many of the same design features and organisational strategies at their new home as the previous hell hole they escaped from....
Obviously they see you and start screaming, "Oh Armok, not again!"
Yeah, we could rationalise it by saying they never fully realised their trauma because they were too busy staying alive and getting somewhere safe and only once they arrived it really hit home.
Still sort of annoying when the resident cat ladies of an old fort migrate to your new place, because no matter how many in-game years have passed since you abandoned the old place, they'll arrive with the full unhappiness hit for losing every last one of their cats. Better remove all your mayor's/expedition leader's labours, because she/he will have to do a fair bit of pacifying and consoling to keep them stable until the grieving period is over.
Anyway: my most challenging successful embark so far - glacier bordering on arctic evil sea. While the miners are busy carving out the starting dormitory and dining room, i get an interruption message from wildlife. Zombie Harp Seals. Hm, what on earth are harp seals? "A small marine mammal", the description says. Well, everybody, you're a wrestler squad now, attack! Needless to say, there are no survivors on the surface.
The two miners manage to carve out some basic rooms, take on the various 'getting started' duties and tick along at unhappy to very unhappy, until one tantrums and punches her colleague, insta-braining him. For some reason she refuses to acknowledge the presence of the corpse lying around in plain sight but still keeps it together until migrants arrive, bury the dead and get the fortress proper going. I later find out that zombie harp seals are quite capable of taking down yaks...
After three years, when i'm getting ready to wrap up business, it's a nice little glacier fort with no fortress deaths apart from the first settlers. Still ended up with an unmemorable ghostly caravan guard...