The reason that UPS will sometimes use U Haul trucks is because either they have had a break down with their UPS truck or they have too many packages to deliver at the time and use the U HAUL trucks for extra capacity.
It's normal to see UPS delivering packages using U Haul Trucks especially during the holidays such as Christmas, Thanksgiving etc.
UPS has been renting U Haul Trucks when they need more delivery trucks to get all the packages they have delivered on time.
It's cheaper for UPS to rent trucks from U Haul when needed than to keep extra UPS Trucks available and just sitting when they don't need them.
I've had UPS deliver packages to me before using U Haul Trucks and I asked the driver about it and that's what he told me.
He told me that they rent the U Haul Trucks when they need extra vehicles to do the deliveries.