Inbreeding can do this. Doesn't matter if you didn't do the inbreeding, all that is necessary is for both parents to be carrying a recessive gene that matches up in some offspring.
As a kid I too was taught the line about birds being unable to walk backwards --- so much junk science is recirculated every year, even stuff that's been disproven for decades! Shameful.
This backwards walking thing is now a trait in some strains of quail etc because some people don't see anything wrong with it if it doesn't kill them outright or in an obvious manner. So they keep breeding them. I would avoid breeding from them personally, which is an issue with rare birds, not necessarily an easy fix problem there. It's a temporary spasm that occurs regularly over their lifetimes, it can be triggered by various things as far as I've heard. It seems to affect the neck in many cases too, they hunch it into their shoulders or it goes under their bodies. I think it may also be a symptom of some kind of poisoning in other cases.
Walking backwards as a symptom also occurs in various others species as a reaction to physical pain of a certain type/location, like walking in circles, or continually heading off to one side, etc. Generally it represents neurological damage, either inherited or acquired.