Here is another example of the use of the term 'impermanent' from SN 12. 20. Bhikkkhu Bodhi - The Connected Discourses - page 551.
"And what, bhikkhus, are the dependently arisen phenomena? Aging-and-death, bhikkhus, is impermanent, conditioned, dependently arisen, subject to destruction, vanishing, fading away, and cessation.
Birth is impermanent ...
Existence is impermanent ....
(and so forth ... down to ...)
Ignorance is impermanent ... and cessation."
This seems to be speaking not about what arises this moment, or how it arises this moment, but about all those things which have arisen over the last twenty years or so, and which have not yet ceased.
Is there a 'heaping-up', 'amassing' or 'accumulation' of such things?
Is there a collection of thousands of things that we are clinging to?
If so, then these things can only vanish in some present moment. Although they arose in the past in one sense, they persist because they arise again each moment in another sense of arise. So 'things' are 'activities' when seen from another point of view [ This only applies to mentally constructed 'things'].
Best wishes, Vincent.