My guess, fwiw, is that Unionism saw backing Brexit as an assertion of their British identity.
But this is mismatched with developments in Conservative Party, which is now English nationalist in outlook, captured by a UKIP tendency.
Hence, Conservatives didn’t think much about the consequences of Brexit for the Union (see also Scotland) and didn’t hesitate to agree to a border in the Irish Sea (even if they are now trying to undo it).
As I’ve noted, the GFA institutionalised a need for cross community consensus. The choices created by Brexit *inevitably* takes NI back to majoritarian, winner-takes-all politics (most obviously in the “where do you put the border?” question).
twitter.com/nickanstead/status/1050006118729207808?s=21&t=jHZImBkXobergPXNiWqNIQ