So, I'm looking on a5e.tools as I don't have hardcovers or pdfs at hand atm, and I haven't found a precise paragraph yet.
However I'd say that if the trick of moving towards a character could trigger Spirit Guardians because the target "enters" the area, then by the same rationale one character with Polearm Savant could
make an opportunity attack while moving towards a target because the target "enters" the threatened area (instead of the target moving towards the Polearm Savant, as usual).
Also, a character could
make an attack of opportunity while moving away from an enemy because the enemy "leaves" the threatened area but didn't disengage.
Both of these are obviously not right, so relative movement doesn't seem to be a thing in this context: what matter is who is actually moving during the action. By extension (and until I find an official rule) I think this would also exclude zapping around the battlefield and having spirit guardians deal AOE damage (effectively covering an area larger than the area of the spell itself!) with no save and without even spending an action or reaction.
I'll keep looking for this specific thing on the PDF when I have time, it bugs me to be unable to find a direct explanation for how "on enter" conditions work. If there's one thing I really passionately dislike about 5e is its departure from a keywords system as we had in 3.5.
Also, not directly relevant as it's for 5e, but there's this
moonbeam question at pag 19 of Sage Advice
However I have to partially correct myself, as it seems that the voluntarity of movement seems to be important only for attacks of opportunity but not for hazards: being dragged or pushed into a hazard does seem to trigger the "on enter" clause, but being forcefully pushed out of a threatened area doesn't trigger attacks of opportunity. I'm not fond of this asymmetry TBH.