One cause would be convenient. The published wording is more careful than that, and the careful version is the one to read.
The cause that is named
AAOS OrthoInfo says bunions may be caused by wearing poorly fitting shoes, in particular shoes with a narrow, pointed toe box that forces the toes into an unnatural position.
Reading the verb
The sentence says "may be caused by." That is a possible cause, not a promise that shoes explain every bunion. It also names a specific kind of shoe, the narrow, pointed toe box, rather than shoes in general. If your shoes do not fit that description, the sentence does not point at them.
For deeper reading, there are four places to go: Bunions and Their Treatment in PubMed Central, the NHS page on bunions, the Royal National Orthopaedic Hospital patient guide to bunions and lesser toe deformities and a report from the UK National Hallux Valgus Think Tank.
What to ask a foot doctor
Bring the shoes you wear most and ask the foot doctor to look at them next to the foot. A bunion visit in Itasca or any nearby village is more useful when the doctor sees the footwear and not only the toe.
The word in front of the cause
The clause that gets skipped is "poorly fitting." The sentence is about fit, and a shoe can be a good shoe and still fit a particular foot badly. Judge the shoe against your own foot, not against its price or its label.