Age

Skin healing is slower in older people while younger skin tends to "overheal", forming larger, thicker scars. [1]

Genetic factors - skin type

People of African and Asian descent, with highly pigmented skin, are especially prone to abnormal scars, such as Keloids. [1-3]

Scar location

Scars over or near muscles that are particularly active, such as the back, legs, shoulders and joints, often spread or become more visible than scars formed on less active areas. [1]

Wound infection or complications

Wound infection increases the likelihood of abnormal scarring.

References:
1. English RS, Shenefelt PD. Dermatol Surg 1999; 25:631-638
2. Brissett AE, Sherris DA. Facial Plast Surg 2001; 17:4
3. Bayat A et al. BMJ 2003; 326:88–92