Whether or not that's the main reason isn't clear -- the government doesn't exactly have a clean track record. Bangladesh temporarily shut off mobile internet access in August in an ostensible bid to curb violence at protests over road safety, but there's evidence suggesting that it wanted to stop the dissemination of media showing pro-government activists and police attacking protesters. In 2015, it also clamped down on individual messaging apps in an attempt to thwart protests.
This latest move might help prevent violence and misinformation, but it could also hinder the opposition's ability to rally support. Prime Minister Sheikh Hasina's administration has been accused of intimidating and imprisoning critics to help maintain power, and this could be another extension of that strategy.