They would do something like that if they were moving something in or out that needed to be off the radar. Could be people as well, workmen for example.
Sunday morning, so likely not many inside, and block access to happenstance onlookers on the outside.
Why would a bomb be planted that for a Sunday morning, anyway? It can't be that easy to get near the Capitol with it, either. Maybe a FF drop meant to be discovered and be to blamed on Trump supporters.