Apparently the attackers were educated at an English immersion school of some sort. Spoken Bengali, like British English, is very sharply differentiated by class. The menial workers at the bakery were apparently surprised at the obviously upper middle class speech patterns of the terrorists, as well as the fact that some of them spoke casually in English.
This is not surprising. Terrorist ideologues are often from privileged backgrounds.Marc Sageman has presented the most thorough ethnographies that I know of, and they’re rather clear. Generally middle to upper class, often with a technical background. Also a disproportionate number of converts. Many people with cosmopolitan backgrounds, in term of having exposure to other cultures and traveling.
As for why radicalism is cropping up in Bangladesh, it’s because they know that they’re in the middle of a culture war they might lose. There are atheists and gays in most countries, but in Bangladesh they are starting to be public. That’s a sign that the norms of conservative Islam are breaking down. Additionally, economic development and NGO influence are also integrating the nation into a web of international commerce.