What could cause cellulitis to reoccur?

Reinfected. Usually it is re-infection not re-occurrence. For example, when a person developed cellulitis on lower legs, he/she might have fungal infection involving feet, toe webs or toes. The fungal infection causes open wound in those areas. Later, bactiriae might enter these channels, reaching lower legs and developing the cellulitis again. The fungal infection should be treated in the first palce.
Whats the cause?? Cellulitis is basically a visible inflammation (itis) of the skin and soft tissues (red, hot, swollen and tender). It can be caused by many things including sunburn (radiation), trauma (sprained ankle) or poor vein circulation (stasis dermatitis). It need not be caused by bacteria. If you have repeated episodes of this, get to a wound care or vascular specialist, the diagnoses you have is wrong.
Cellulitis reinfects. Reinfection from cellulitis may result from a small opening in the skin that gets reinfected from a simple scratch from your own nails! Washing the legs with hibiclens or a similar cleaning agent, can prevent reinfections from occurring.