Pepper-sprayed customers, smash-and-grab looters and bloody scenes in the shopping aisles. How did Black Friday devolve into this?

As reports of shopping-related violence rolled in last week from Los Angeles to New York, experts say a volatile mix of desperate retailers and cutthroat marketing has hyped the traditional post-Thanksgiving sales to increasingly frenzied levels.

With stores opening earlier, bargain-obsessed shoppers often are sleep-deprived and short-tempered. Arriving in darkness, they also find themselves vulnerable to muggers.

Add in the online-coupon phenomenon, which feeds the psychological hunger for finding impossible bargains, and you’ve got a recipe for trouble, said Theresa Williams, a marketing professor at Indiana University.

“These are people who should know better and have enough stuff already,” Williams said. “What’s going to be next year, everybody getting Tasered?”

Across the country on Thursday and Friday, there were scenes that shocked:

A woman pepper-sprayed 20 other customers at a Los Angeles-area Walmart in an attempt to get at a crate of Xbox video game consoles.

In Kinston, N.C., a security guard also pepper-sprayed customers seeking electronics before the start of a midnight sale.

In New York, crowds reportedly looted a clothing store in Soho.

At a Walmart near Phoenix, a man was bloodied while being subdued by a police officer on suspicion of shoplifting a video game.

There was a shooting outside a store in San Leandro, Calif., and shots fired at a mall in Fayetteville, N.C.

Nationally, of course, the violent incidents were few and far between during what has become the largest and busiest shopping day of the year.

But they took many by surprise.

“The difference this year is that instead of a nice sweater you need a bulletproof vest and goggles,” said Betty Thomas, 52, who was shopping Saturday with her sisters and a niece at Crabtree Valley Mall in Raleigh, N.C.

All black humor aside, however, marketing researchers can cite specific reasons for such behavior.

Black Friday has become more intense as companies compete for customers in a weak economy, said Jacob Jacoby, an expert on consumer behavior at New York University.

The idea of luring in customers with a few “doorbuster” deals has long been a staple of the post-Thanksgiving sales. But now stores are opening earlier, and those deals are getting more extreme, he said.

“There’s an awful lot of psychology going on here,” Jacoby said. “There’s the notion of scarcity — when something’s scarce, it’s more valued. And a resource that can be very scarce is time: If you don’t get there in time, it’s going to be gone.”

But there’s also a new factor, Williams said: the rise of coupon websites like Groupon and LivingSocial, the online equivalents of doorbusters, which usually deliver a single, one-day offer with savings of up to 80 percent on museum tickets, photo portraits, yoga classes and the like.

The services encourage impulse buying and an obsession with bargains, Williams said, while also getting businesses hooked on quick infusions of customers.

“The whole notion of getting a deal, that’s all we’ve seen for the last two years,” Williams said. “It’s about stimulating consumers’ quick reactions. How do we get their attention quickly? How do we create cash flow for today?”

To grab customers first, some stores are opening late on Thanksgiving Day, turning bargain-hunting from an early-morning activity into an all-night slog, said Ed Fox, a marketing professor at Southern Methodist University in Dallas. Midnight shopping puts everyone on edge and also makes shoppers targets for muggers, he said.

In fact, robbery appeared to be the motive behind the shooting in San Leandro. Police said robbers shot a victim as he walked to a car with his purchases around 1:45 a.m. Friday.

“There are so many hours now where people are shopping in the darkness that it provides cover for people who are going to try to steal or rob those who are out in numbers,” Fox said.