I read again but still don't know what's the trigger for PP expansion or retreat.
Powerplay is not the same as the BGS.
In Powerplay you have 'bubbles' that have a central control system and a bubble of exploited systems around it. When these bubbles overlap the overlapping area becomes contested if each bubble belongs to different powers. Contested systems do not generate CC (command capital). The same holds true for overlaps within the same powers territory (loss of CC).
Sometimes powers with lots of CC do this to drain another power of CC. This is Weaponized Expansion.
To force a power to lose bubbles you need to undermine them, lowering CC to a point where the power cannot afford to keep the systems they control. Generally the more distant go first.
You have to be pledged to undermine a power.
As an independent pilot you can 'flip' systems instead though. Each power is strong or weak against certain government types. If enough exploited systems in a bubble swap to either a favourable or unfavourable government the the amount needed to fortify goes up or down. So a neutral starting trigger (for example) can be 5000, with favourable being 2500 and unfavourable being 7500.
By making more work to do, you raise the chances (sometimes) that another power cannot fortify everything and make them more vulnerable.