Diabetes particularly predisposes to developing a foot ulcer because of the changes which occur in the feet with neuropathy. This means nerve damage. High glucose levels are associated with malfunction of all nerves. In the feet this is noticeable in various ways but it is the combination of these changes that greatly increase the risk of infection and ulceration.
Nerves controlling muscle strength will be weakened and this results in an altered foot shape. The stresses and pressures exerted through the foot on walking may then focus on one area. This hot spot will be the site of a developing ulcer just due to pressure but combined with sensory loss this will happen even without the person knowing. Without knowledge of this happening more walking causes more damage.
The skin of the foot also fails as a protective barrier to infection. The autonomic neuropathy component of the problem alters skin blood flow and sweat production. Infection is much more likely in this scenario. The situation is now sufficiently primed for impending disaster, a pressure ulcer forms by trauma, the persons foot is insensitive to pain, the skin cracks and infection creeps in and starts brewing. This is exactly how many diabetic foot ulcers form.