It's nearly always crowded there late at night... I'm not sure what it is that makes so many people visit at night.
It must be hard to have a father on that wall, and perhaps the pain is more difficult at night.
>>I’m not sure what it is that makes so many people visit at night<<
Partly boredom; if you’re there for a few days, and stuck in your hotel room, there’s not much to do at night; plus, the tourist thinks they are *beating-the-crowds* by a evening visit.
The type of person who did this is someone who wouldn’t mind burning an American flag, throw litter on the ground, or sympathizing with the *occupy-crowd*. Pox on them!
Many of the monuments are much more dramatic at night. Especially the Korean War and FDR memorials.
However, the biggest reason is probably the weather. DC is hot and humid this time of year. But, it's tolerable at night.