#BlackLivesMatter has an implied "too".
#AllLivesMatter has an implied "except black ones"
That's kinda how I see it. #BlackLivesMatter is a statement because, YES, while all lives DO matter, we are in a time where Black lives are treated LESS that most other races. This is not a statement for all people to piggyback on, this is in support that, yes, Black lives matter, they have worth and are precious. They should not be treated as lower-class citizens.
#AllLivesMatter, I can just throw a guess that it was started by some butt-hurt white person that was disgusted in seeing another race, for once, get some attention OVER whites. See, when whites see #BlackLivesMatter, they are so used to the universe revolving around them that seeing ANYTHING that leaves them "out" of the fold is crushing to their entire being. By saying "#AllLivesMatter", at least they could be included into the mix, and feel even more special about themselves. "See, WE are part of it. ALL lives matter, including US.... because we're WHITE, so OF COURSE we matter!! Don't say we don't, because the world tells us WE DO! Wah!!!".
While, truthfully, all lives do matter(lives matter), this hashtag doesn't mean that. Just white whining about how people have the nerve of not including them into something(because they are the fuckin' "shining stars of existence", therefore, by default, SHOULD MATTER... more that other races).
I think you could tell the ego-centric whites from the others. The good ones don't hashtag #AllLivesMatter. Only someone privileged enough to feel their ego and worth being challenged does that.