I’m betting on some language or accent just sort of dropping/mashing syllables together and that pronunciation just kind of catching on or getting passed along to someone else who further mangles to pronunciation until it finally ended up at bike.

Personally, as a Philadelphian, when I say “bicycle” I’m usually not fully saying 3 syllables like “bi-sih-kul” it usually comes out more like “bise-kul”

Take that a step further to maybe something like “bi-kul” and then finally onto “bike”