Everything is glued down this month — why do colonies propolise so hard in late summer?

Quebec, 5 years, and every August it's the same: inspections that were smooth in June turn into a demolition job. Frames cemented to the rests, inner cover practically welded on, and yesterday I bent my lighter hive tool getting into my strongest colony. My gloves are permanently amber at this point. I understand roughly that they're weatherproofing for winter, but I'd love the fuller picture — is it temperature-driven, daylight-driven, forage-driven (the poplars and conifers here are oozing resin right now)? And more practically: any technique for freeing badly glued frames without the lurch-and-squash when they suddenly let go? I've started twisting the hive tool instead of levering and it helps, but there must be more tricks. Also is it true chilled propolis snaps clean? My inspections are usually warm-afternoon ones so everything is at maximum chewing-gum consistency.