Like why are norwegians of all people going into bangladesh?
As for some of the other countries... Bangladesh has pretty close ties with Southeast Asian nations, especially Malaysia where there are a lot of Bangladeshi expats. I suppose they send people to Bangladesh as well.
China has funded a lot of infrastructure projects in Bangladesh lately and often sends construction workers, engineers, and supervisors to oversee the projects. Of the foreigners I've seen in Bangladesh, most are Chinese.
The US and UK have large Bangladeshi diasporas and it's possible some of these have migrated back (along with NGO workers like Norway).
Those are just my guesses though. I was taken aback by some of these answers too.