I'm guessing the fact that there's uncertainty to say the least about ensuring the supply of food, fuel, medicine and other goods that are imported. As well as the possibility of a large number of companies deciding that it's not worth continuing to operate in the unknown environment of a no deal Brexit UK when they could just relocate to the EU and carry on operating as they are, which could easily lead to large scale job losses.

Any one of those situations is probably enough to make them consider the possibility of martial law, when you've got all of those, as well as a good chance of last minute protests and counter protests, you'll want to dust off the plans they have for how to enact martial law.