Might be maximizing slots for overseas And long haul/transcon departures by the airport. These flights arrive in the early to mid afternoon so connections from IAD would get these passengers out by 6p. Inbound connections for outbound overseas flights would also be maxed between 4/5/6p. Unlike US, UA uses IAD as an overseas and transcon gateway/hub with larger aircraft and thus must concentrate its slots accordingly.

While there are local businesses that could generate mid-evening domestic departures perhaps UA sees more such departures from DCA and sends it's planes there for departures between 6p and 10p?