Decoding a mystery sequence of falling boulders

I’m trying to unravel the algorithm for an old 1980s game called Wanderer. The game is a simple grid: 16 cells high (all visible in the image) and 40 cells wide (though not all are depicted). In the game lots of things happen, but I’m only concerned with an sequence of falling boulders. They fall in a sequence governed by specific rules as to how they can be triggered and how triggers are priorised. The full sequence is triggered by a single move by the player, and there are definitely no other influences at work – it’s a self contained system. Here is the before and after layout of the player’s one move (to the left):

The boulders tumble in the following sequence:

I am trying to understand the logic of this sequence.

Some notes:

1. Boulders only move when one of the following rules are satisfied:
   1. the cell below them is unoccupied,
   2. the cell below is a slide (shown white in the screenshots), the next cell on the opposite side from the slide ‘wall’ is unoccupied, as well as the cell below that – they essentially just slide down the wall,
   3. they can also role off boulders if a way to slide off them to one side, but in this scenario it only happens at the bottom of the screen so we can ignore this rule.
2. When any boulder starts falling nothing else moves until that boulder has stopped at the bottom.
3. In the game the player can trigger boulders 1 column sideways and 2 rows above (8 cells in total) when they move away from the boulders when underneath or alongside them. These positions are relative to the players initial position, not after they have moved. It is possible the player’s move in this scanario triggered just the bouder next to it, or alternatively up to 4 boulders (numbered 1,2,3,5). It is unclear to me how many boulders were triggered by the fall of boulder #1.
4. There may be a queue in operation, but it’s unclear how boulders are prioritised to join it, or whether they can join when still unable to move at that moment, or only once they are potentially able to move.
5. There is an interesting spatial repetition in the 2-5 and 6-9 sequences.

If anyone is able to resolve this problem by identifying rules that unambiguously explain this seqience they get 100 genius points and my deep respect. I think I’ve framed this question unambiguously, but if there are any clarifications please ask.

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Edit: here’s a gif of the scenario using an old Wandroid app (Android) now unavailable from Google Play but still available from some sites.