Why do I have cellulite at the back of my legs after a butt fat transfer?

Hi there-
It is very unlikely that the appearance of cellulite was caused by the surgery, as the procedure you had involved ADDING fat to the area, so that if anything the appearance of dimples should have diminished...
Because cellulite is fundamentally a skin issue, it is most likely that you had the cellulite before the operation, but because you never spent as much time scrutinizing your behind before surgery as you do now (human nature, of course), you simply weren't as aware of the problem as you are now.

A good candidate has enough fat available for transfer in the procedure. The amount of fat that can be transferred in a BBL depends on how much fat the patient has to harvest and the amount of space there is to move the fat into. It is recommended for the patient to have a BMI of 30 or less. If you are too thin, a Brazilian Butt Lift may not be possible.

The two are unrelated.  Butt lift involves remove fat from different areas of your body and then injecting the fat into the gluteal area.  It should not affect the back of your legs.  It is possible that they were there but went unnoticed.