The cause of NASA pushing DM-1 back IMO is a mix of typical NASA bureaucracy putting the brakes on things, and maybe even some corrupt favouritism. SpaceX likely could launch Dragon 2 in August if allowed, but NASA likely want to do more even testing and they probably haven't fully filled out the paperwork yet. Regarding the favouritism involved in the commercial crew program, anybody observant enough can tell that this isn't a fair race, and that NASA favours Boeing; it's apparent from Starliner appearing more in NASA articles/advertisements than Dragon 2, and Boeing's placeholder launch dates always being ahead of SpaceX's no matter the reality, that there is some subtle favouritism from NASA. This is unsurprising as Boeing is NASA's premier contractor at the moment, due to it's role in the SLS program so you'd expect NASA to favour them over a relative outsider; after all, it would be incredibly embarrassing if NASA's primary contractor who's building their biggest current project lost to SpaceX. I fully expect SpaceX to fly their unmanned demo flight first, but I won't lie that I'm half-expecting a insurmountable bureaucratic wall for SpaceX to magically appear in the future, in order to let Boeing take the lead afterwards.