Why do people have fetishes, such as foot fetishes?
How do such fetishes form? Are childhood incidents the main cause of fetishes? Are fetishes a sexual disorder?
A fetish is arousal from an inanimate/non-living/non-sentient object. They fall into two categories: media fetish (e.g., a fetish for leather, regardless of the form it takes), and form fetish (e.g., a shoe fetish, regardless of the material the shoe is made out of). There are multiple theories on how fetishes develop:
What you are asking about is actually referred to as Partialism - arousal from a specific body part :)
I was unable to find any reliable explanations of how partialism develops, but I can confirm that neither partialism or fetishism are considered abnormal unless they cause significant impairment or distress to the individual, or cause issues with the individual's lawful functioning in society. Either would be considered a sexual disorder if that criteria is met, otherwise they would not be classified under the DSM as a disorder. Generally, in order for this criteria to be met, the fetish would need to be rather prevalent, so, by that definition, someone simply having a private preference which does not interfere with normal day-to-day functioning would not be seen as having a sexual disorder.