Since the ones being delayed are the 70+lb boxes and happened numerous times, I would say that UPS is not giving enough time to transport irregs to the correct plane, be it from late planes, or just bad scheduling of arrivals and departures.
If I am thinking correctly, I've noticed that irregs are not subject to day guarantees on ground packages, not sure about air. Don't know for sure without tracking, internal tracking gives a lot more info than the website.