Usually on UA when theres a direct flight to/from an international destination, they NEVER use the same aircraft. For example, UA943 CDG-ORD-SEA the CDG-ORD is a 777 then there's a plane change ORD-SEA being another type of a/c usually a 757 767 A319/20. Another good example UA815 ORD-LAX-SYD, ORD-LAX is a 757 or A319 the LAX-SYD is 747-400. Airlines have direct flights, which means one stop and possible plane change, because of marketing reason.
Most likely, when the flight was suppose to arrive from ORD to SEA, it will turn for another flight, therefore, it CANNOT possibly wait for misconnects that long.
Aircraft servicing in UA computer mean "mechanical problems." Furthermore, it is most likely that something was wrong with that DC10, since it's a very old aircraft. Another reason that might cause the delay for yesterday's flight is because ORD has ATC problems. I know that yesterday, ORD was on a ground stop, which means the airport was closed for awhile due to the congestion and the weather condition in ORD. When that happens, it screws up a lot of flights because the inbound a/c that will turn for another flight will be arriving late.
I hope this helps.