I will have a go
As far as the EU is concerned, Northern Ireland is part of the UK. If Britain is left with no-deal after the Brexit deadline passes, Northern Ireland is in the same boat.
The Republic of Ireland are part of the EU in their own right as a sovereign nation. So if Northern Ireland fell back on WTO rules, a new customs barrier would suddenly spring up across the Ireland-Northern Ireland border.
This creates a big issue. The Good Friday Agreement dictates that people and trade must flow freely between Northern and the Republic of Ireland.
A no-deal would break that agreement, and there would suddenly be a more palpable division between the north and south.
The Good Friday Agreement was instrumental in ending the violence in Ireland between Nationalists and Unionists. There are some that fear a customs check could reignite violence as the choice of remaining part of the UK becomes more divisive once again.
The WA "back stop" protected the GFA, much lessened the threat of a resumption of violence and bought time until technology removed the need for a hard border. That technology does not exist yet
In Northern Ireland, the Unionist and Nationalist communities living under the GFA have not integrated, they live in a permanent state of stand-off
I quote from Dearbhail McDonald who said in this article
https://www.theguardian.com/commentisfree/2019/aug/02/british-no-deal-brexit-irish-peace-unity"What was once an entirely fringe aspiration for nationalists, and a dreaded fear for unionists – each side comforted by the thought that the conversation about unification was decades away – has become an urgent debate for all."
and then
"Unification without the support of unionists, whose traditions and identity must be protected as part of any shared future, cannot be pursued as a zero-sum game. Otherwise we risk repeating history: the Troubles in reverse. Northern Ireland could not, in its current perilous state, survive either a hard Brexit or a united Ireland. Its communities are suspended in an in-between space, in which they have enjoyed 21 years without violence but have not yet progressed to a positive peace or meaningful integration – a process that takes time and effort to heal wounds."