1. The reason that the PS starts with “avijjā paccayā saṅkhāra” is that avijjā is the root cause.
– If one fully understands PS (an Arahant), one would have fully understood how future suffering arises by generating abhisaṅkhāra due to avijjā. That means an Arahant’s mind would not attach to any arammana regardless of how enticing it is.
However, no one acts with avijjā at all times. Even the worst immoral person may engage in moral deeds sometimes.
– Thus, avijjā remains “hidden” most of the time. The Pali word is “anusaya.”
That avijjā comes to surface when we get attached to an enticing sensory input (arammana).
– That is why the PS is normally TRIGGERED by sensory input (arammana).
That initial attachment is tanha.
– Upon attaching, the mind will start trying to “get more of that arammana” by “keeping that arammana close”. That is the “tanha paccaya upadana” step.
– That is when one starts acting with avijja by thinking defiled thoughts (mano and vaci abhisankhara), speaking, and doing unwise things (vaci and kaya abhisankhara).
– Then the other steps will follow: “sankhara paccaya vinnana“, “vinnana paccaya namarupa“, etc.
2. Sometimes it may not be clear that avijjā has been triggered by an arammana.
– It is clear that we get attached to things that we see, hear, taste, smell, or touch (five physical senses). However, there is a sixth way that an arammana comes in and that is not always evident.
– That sixth way is for an arammana to come directly to the mind. That is the case when we had ALREADY formed an expectation (i.e., had formed an attachment) with an arammana that came through the five physical senses sometime back. For example, one may have seen an attractive person a few days ago and had formed an expectation to date that person. Then that can come back to the mind as an arammana at any time.
– Details on that at, “Chachakka Sutta – Six Types of Vipāka Viññāna“. That is a bit deeper post. The point is that an arammana can come in via one of the six senses as a vipaka vinnana. So, another way to say is: that the PS process is triggered by a vipaka vinnana. There are different ways to express the same concept.
3. The reason that I wrote the post that you brought up is the following. Sometime back, Upekkha100 asked about the difference between tanha and upadana at this discussion forum. It could be useful to read that discussion: “Difference between Tanha and Upadana”