Nothing to do with the large number of pies eaten in Wigan but to do with a miner's strike in the 1920's when all the pits in WIgan and Leigh went on strike. The Wiganers went back and broke the strike, the "Leythers" continued to strike and got better pay and conditions. Therefore the Wiganer's had to "eat humble pie"... so it's a term of abuse from Leythers to Wiganers. The hostility between the two towns stretches back hundreds of years to the civil war when Wigan was Royalists and Leigh were Parliamentarian and continues to this day.
"This will be the 'pie-eaters' last year in the Premiership"
Also liable to leave from somewhere 20 minutes early and complain that they have been cheated out of something. This is known as "The Wigan Walk"
'We're away to the pie-eaters next Sunday'