think there are a number of influences behind this. first, "we outside" came out of a broader slogan "we outside hoe" which was promoted via southern hip hop. from what i've always understood, "we outside hoe" was a taunt, a sorta admonishment to one's oppositions or rivals, to come see you because you outside and not ducked off somewhere in the house or wherever fearful. if the opposition or rival has been making threats, you want to see how good they are on carrying out that threat. and even more implicity, the very nature of you being outside means that you don't view the opp or rival as posing any significant threat you can't defeat. so at its root, it's a posture declaration.
post 2010's the 'hoe' in the slogan seemed to have been dropped giving it a softer tone depending on context. it can still be used in the original context - which is how i initially interpret it, or it could be an exclamation of jubilation. you see this a lot with women. a woman sees another woman dressed to the nines - she exclaims "you better work", a woman is listening to a minister deliver a deeply riveting sermon - she exclaims "you better preach", a woman goes out with her girlfriends and in a moment of happiness, freedom, connectedness, whatever it means for them in that moment, she exclaims "we outside."
black people have always jubilated even the most mundane things, because historically, we were both restricted from doing the most mundane things as well as freely expressing our emotions.
to understand why people are saying this would require a cultural intimacy that those on the outside are just going to miss.