It's a very good question. It's worth remembering that in all of its history, NATO only invoked article 5 once and that was following the 9/11 attacks on the United States. Article 4 has been invoked nine times so far, five times by Turkey following events around its borders, a number of times by European allies following the invasion in 2022, and then of course we have Poland and Estonia this week.
You asked the question, what are the triggers? The beauty of article 5 is that it's pretty clear in one way, in the treaty, that it's an armed attack against an ally. At the same time, it's also a little bit fuzzy in that it's an armed attack against an ally, but that's not defined.
There are advantages of having it that way to ensure that our adversaries don't know exactly where the threshold is.