I believe the general logic is their stature/ size. A lot of the ones that get Pressure naturally are legendary Pokemon. As for large Pokemon, Wailord is the main one (I think anyone would feel the pressure when facing a gigantic whale...).
Another interpretation could be the intensity of the situation the Pokemon present. The non-lengendaries are Spiritomb, Weavile, Bisharp, Absol, Vespiquen, Dusknoir/ Duskclops and Aerodactly.
From their dex entries:
- Vespiquen & Bisharp are noted for leading packs and commanding their lower evolution forms
- Dusknoir/ Duskclops are linked with the spirit realm
- Weavile travel in packs to hunt
- Absol's appearance foretells danger/ disaster
- Aerodactyl is known for its viciousness
- Spiritomb was originally 108 spirits bound by a keystone as a punishment (I'd imagine the pressure is pretty intense in that keystone...)
That's the logic.