That said, these normally neutral NPCs got a massive AI upgrade recently, and the approximation that it came in around the Bikers update is probably about right. They display behavior that clearly isn't a product of chance, nor is it similar to how they behaved months ago. This can't just be chalked up to differences between the old-gen and current-gen versions either, since older iterations of the Enhanced Edition were free of these murderous mobs as well.
The strangest thing about this is that as far as those of us not part of the Rockstar dev team can tell, there is no cause or trace of this change. Data-miners, who usually dig through GTA Online code and game files to uncover what the next unannounced DLC is, have tried to pinpoint the cause of this increased aggression.
However, the people who have uncovered and leaked all of Rockstar's secrets consistently for years found absolutely nothing. Not one reference, not one code entry, that would explain this. In their own article Kotaku mentioned that they contacted one renowned (read: not full of shit; actually leaks legit stuff) data-miner TezFunZ2 for clarification.
As it turns out, data-miners don't exactly dig too deep. Instead of heaving rock out of the figurative earth in this analogy, data-miners only scrape a bit of soil up to reveal the so-called "tunables" of the game which indicate content changes coming. This is how they get a hold of names, scripts and 3D models that can be loaded into editors and viewed. What these don't reveal, however, are the inner workings and underlying programming of the game.
Naturally, this being the GTA community and the GTA community constantly blaming Rockstar and their conceived 'greed' for absolutely bloody everything, the most common theory is that this increased aggression is a conscious design decision on Rockstar's part.
Now, from a persistent game-world perspective, this would actually be a stroke of genius. If you think about it, players have been blowing shit up, stealing, killing, robbing, murdering, battering, assaulting and performing all kinds of other illegal activities in the virtual city of Los Santos for three years now. Naturally the NPCs are getting pissed off by these folks wrecking their city, so it would make a whole lot of meta-game sense for their AI to become more aggressive over time.
Of course, instead of seeing this sublime but masterful scenario, players think Rockstar did it so they have to pay more for repairs and whatnot, making it harder to build wealth, making Shark Cards more attractive, and therefore increasing their sales. Now, this conspiracy theory might seem attractive to those who get off on antagonizing AAA developers, but considering Shark Cards were selling like hotcakes before this phenomenon began, so it's not like they need this underhanded tactic to drive sales up marginally.
What is a more likely explanation is that the plethora of bugfixes that went into the game over the course of several patches included one minor change that catalyzed a butterfly effect of sorts but in code. Programming such a massive and complex game like GTA 5 is tricky business, and so many things are interconnected that fixing one bug might lead to three others. Or thirty.
Maybe something a Rockstar engineer fixed ended up changing something else which changed something else which changed something else again and boom, suddenly every NPC in GTA Online is a ravenous murderer. Of course, anyone remotely familiar with coding a game will know that that's a vast simplification of the situation, but essentially it's correct.
Whatever the cause of this virtual rage-plague is, there's no sign that it's going to change anytime soon. Players will have to exercise great caution whenever they play GTA Online. The NPCs are always watching, always alert. No place is safe, no PED harmless. Stay safe everyone.
