UGS and 1Ks beat a CP status on a UA flight. Flying through IAD introduces a high likelihood of one or more 1Ks involved in any waitlist, particularly on the IAD-LGA flights. Your "almost certainly would have made" comments don't hold much water IMO, you don't know who, if anyone, made it off the standby list and there is no way you'd know their status' and why they might have cleared before you (its more than just status, it's also which waitlist). Also, if you were flying to LGA, you had EWR and JFK as possible alternatives, so it wasn't a go/no go decision just because you didn't get the flight you wanted. You could have just cancelled the entire itinerary when you removed yourself from the second flight and gotten a credit less the change fee, the return flights would've been more expensive as you would have now been buying one ways, not a RT and it would've been shorter notice.

I agree with jan's post above.