The same thing is happening that has always happened. Neighborhoods turn bad, drop down the socio-economic scale and the people who live there become enraged. Immigrants (illegal and otherwise) arrive and start shooting up the streets. It's been happening for 150 years.
Probably longer. The Five Points area of NY was a notorious Irish criminal zone in the 1830s. Downtown/Central LA has always been terrible - it's one fleeting moment of normalcy was pre-WWI.
Starting in the 20s, development of outer fringe LA neighborhoods (eg Beverly Hills) killed inner-city LA. It got so bad that the city had to bulldoze Bunker Hill (where turn of the century mansions had become flop houses) in the 1960s.
South-central is even worse - a giant spread-out Bronx. I almost feel sorry for Branton - he really thought he could turn things around. Problem is, in NYC, only 1 borough is bad; enough 'normal' people live in the other 4 to make reform work.
In LA, except for Pac Palisades, Bel Air & WLA, no one really lives there, nor cares what happens downtown as described in this article. People with means move to OC or up to Ventura (these are red counties, by the way) with a simple mandate to not let LA spread.