The Dictionary of Slang and Unconventional English by Eric Partridge explains socker, or soccer, as: "By truncated assoc. + Oxford -er".
The habit of familiarising words by truncation, mutation and the addition of -er began, according to Partridge, at Rugby School and reached Oxford late in 1875. So Addison's Walk was "Adders" and Jesus College became "Jaggers".
An OED citation, however, shows footer is older than soccer: "A peculiar fashion of their own [at Harrow] which prompts them to call football `footer' " (Boy's Own, 1863).