I had a random red circular spot on the back of my hand and a skin biopsy came back that it was eczema, what would cause it? Why is it one spot?

Unusual to biopsy ec. Eczema is not usually diagnosed this way. The microscopic features of eczema are not specific. Your question is best answered after exam and discussion with a dermatologist. Localized dermatitis may be nummular dermatitis or contact dermatitis, etc.Eczema is a type of dermatitis. Anyway it can be treated.