Diageo’s $40 million global creative review of its marquee Johnnie Walker brand has caught many by surprise, given the sales growth the brand has enjoyed under longtime lead agency Bartle Bogle Hegarty and its "Keep Walking" campaign.

In fact, agency rivals and a search consultant—who typically are cynical and critical of agencies and their work—can’t fathom why the world’s No. 1 scotch whisky is looking around. In addition to BBH, sources identified the finalists as BBDO, Anomaly, Ogilvy & Mather and Wieden + Kennedy.

Observers see "Keep Walking" as a gold standard in spirits advertising and a driver of Johnnie Walker’s growth. When the campaign began in 2000, brand sales represented 13 percent of the global market; at the end of 2013, that figure exceeded 20 percent, according to the IWSR.

"It’s one of the world’s greatest brands. And you know what? BBH made it one of the world’s greatest brands," said a leader at one rival shop, adding, "I don’t like seeing agencies that really built something getting dusted off."

Russel Wohlwerth of External View Consulting Group is equally puzzled by the search. "You never know what goes on behind closed doors," he said. "But on its face, it seems strange they would place the account in review, given the success of the product and the success of the campaign."

Perhaps Diageo is just doing due diligence after 15 years with BBH. After all, even global brand director Guy Escolme admitted that the relationship has been "hugely successful." Nevertheless, Escolme added that "the time is right to invited selected agencies, including BBH to look at how we take the brand forward into the future."