Supported the EU against the UK over the requirement to treat various foodstuffs imported into NI from the UK Mainland (particularly sausages but other items too) as imports from a third country for the purposes of customs and food-standards inspections.
This, it is feared, will lead to many mainland supermarket chains and major exporters stopping exports to NI altogether, and relying on increased exports via the Republic to make up any local shortages.
The Unionists are upset about this because they see it both as a betrayal of promised Johnson made to them before the election, which it is, but at the time this was obvious, and they can't have believed Johnson would honour his promises, since he was making contradictory commitments depending on his audience, and as a further step forward on their inevitable, though highly unwelcome, path to leaving the UK and becoming part of the Republic.
They're on the wrong side of history, and they know it, but they're determined to delay things as much as possible and fight it every step of the way.
They ain't happy, and with good reason, since Johnson took them for suckers by making them impossible promises and then discarding them, just as he's done to his former wives, mistresses and girlfriends when he no longer wants them around, and they're taking it out on anyone who seems to be siding with their enemies in the EU and the Republic.