The fast food times, they are a changing. Once-struggling Wendy's is now poised to surpass Burger King in U.S. sales, knocking the Whopper from its No. 2 perch. Wendy's is expected to notch more than $8.4 billion in sales this year, more than $50 million more than Burger King. Wendy's has far fewer locations, too: The chain has about 5,800, compared to Burger King's 7,200 U.S. stores. McDonald's, of course, remains in the top slot, with more than four times the sales of either Wendy's or Burger King. Still, Wendy's ascent marks the first time the top three burger chains have reordered since 1969. How did Wendy's do it? Here, five theories:

1. A new owner's outreach to customers
"Wendy's fortunes have been revived since investor Nelson Peltz's Triarc Cos. bought the chain in 2008," says Julie Jargon in The Wall Street Journal. Soon after it was acquired, Wendy's began an extensive survey of customers. "They told us they liked the idea of fresh foods with as little processing as possible and ingredients they were familiar with," says a Wendy's spokesman. The 10,000-customer, 18-month survey led to a "reinvention of [Wendy's] core menu."

2. A fancy new menu
Wendy's new menu ditched its basic iceberg lettuce and tomato salad in favor of four new salads with nearly a dozen different greens, and more inventive ingredients like asiago cheese, pecans, and apples. On the french fries front, Wendy's switched from using a mixture of potato varieties to only using Russet potatoes, leaving the skins on, and sprinkling them with sea salt. With the rise of "better burger" chains that use high quality meat, "people are looking for better food now, and ultimately the food is what's going to get people coming back," says Jeff Greenhouse, a marketing consultant.

3. A new shape for burgers
Wendy's success is "thanks in part to a subtle rounding of its signature square patties," says Martha C. White at TIME. It was a "breakthrough" when Wendy's execs realized that patrons thought the square burgers implied that the meat was overly processed and not fresh. The chain didn't want to lose the trademark shape entirely, though, so the company came up with what it calls a "natural square" shape — a thick patty with somewhat uneven edges that give it a handmade look.

4. A focus on past greatness
Wendy's has staged a remake of its 1984 "Where's the beef?" ad campaign. That has helped to remind "customers of its glory days," say Alan Rappeport and David Gelles in The Financial Times.

5. A rival's struggles
"Wendy's has benefited in part from weakness at Burger King," says Jargon. In recent years, the Whopper chain has been on the decline due to various ownership and management changes, and a dependence on young adult customers who are hard hit by unemployment. BK has also failed to sufficiently change its menu with the times. "Flame-broiling and 'Have it your way' are not nearly enough to connect with many of today's sophisticated burger-sector consumers," says restaurant analyst Mark Kalinowski.