Under the Protocol the UK government is responsible for applying checks on goods entering NI from Great Britain to ensure that goods destined for the Republic of Ireland meet EU regulations. This is to avoid a "hard border" between the Republic and NI - i.e. avoid any need for checks at the land border.
Article 16 of the Protocol states that "if the application of this Protocol leads to serious economic, societal or environmental difficulties that are liable to persist, or to diversion of trade, the Union or the United Kingdom may unilaterally take appropriate safeguard measures. Such safeguard measures shall be restricted with regard to their scope and duration to what is strictly necessary in order to remedy the situation. Priority shall be given to such measures as will least disturb the functioning of this Protocol."
It may be noted that "or diversion of trade" is a separate clause. It is not "serious diversion of trade which is liable to persist" but just "diversion of trade" which is a pretty low threshold.
In the months since the Protocol came into force there has clearly been diversion of trade, particularly in the case of agriculture and food, with many Northern Ireland businesses switching suppliers from those in Great Britain to those is the Republic of Ireland. And this has happened notwithstanding that we are still in a "period of grace" before the Protocol has been fully implemented.
With the benefit of hindsight it seems obvious that at the time the Protocol was agreed both parties should have known that the diversion of trade condition in Article 16 would arise almost immediately. So why did they sign? It is one thing to make an agreement which you believe is likely to work but to include a "get out" clause just in case there are difficulties. But why sign an agreement where the conditions allowing the "get out" clause to be used are very likely to arise almost immediately?
So my question is: What could account for the agreement of terms subject to a "get out" conditional on circumstances which are seemingly inevitable.