My theory is the altercation at the store happened. This nikka left, told someone what happened.
Nipsey drops the tweet dissing the dude. Someone gets in his head, like you gon let him diss you like that. Then the rest is history.
I've seen impulsive, weird nikkas like that, someone egged that remedial nikka on and baited him into acting out. He probably wouldn't have done it on his own otherwise.
Nip's tweet was icing on the cake, because while the world wouldn't have known about the incident, a lot of people around where he hangs would have.