What causes a ripple effect after a BBL?

It would be helpful if you included some pictures describing the concerned appearance.
I think you’re referring to cellulite which is made worse with fat transfer.
Cellulite forms in certain parts of the body. It tends to be weight-bearing areas like the back of the thighs and the sitting or pressure point on the buttocks.
These areas have more anchoring connective tissue stabilizing the skin to the underlying muscle fascia.
A good analogy to demonstrate this is the skin on the back of your hand compared to the palm of your hand.
You can tent up or pick up and lift the skin on the back of your hand.
Now try doing that on the middle of your palm.
The palm can handle greater pressure than the back of the hand.
It has lots of connective tissue stabilizing the skin so it doesn’t slide around.
These connective tissue bands are not pliable and don’t stretch as surrounding volumes increase.
When the surrounding volume increases the connective tissue bands get tight and dimples in the skin form.  This is cellulite.
If more fat is added through fat transfer or weight gain the dimpling effect or cellulite gets worse.
These areas  also does not project well with fat  transfer while  the surrounding areas project better because of a lack of connective tissue bands.
People who have a flat spot on the pressure point (inner lower ) on the buttocks tend to have that flat spot remained after a BBL while surrounding areas get increased projection and volume making the flat spot look worse.
People who have these type of conditions are not good candidates for the BBL procedure. This should be reviewed and discussed during in person consultations.
An experienced board-certified plastic surgeon will recognize and anticipate these patients as not being good candidates for the procedure while those with insufficient training or those who lack experience may step in to the pitfall of thinking this can be corrected and look good with fat  transfer.
Being properly assessed to determine the type of outcome you can expect is a key component to delivering consistent high patient satisfaction.
Insist on board-certified plastic surgeons.
Insist on in person consultations.
Choose providers with a proven track record of experience and high patient satisfaction.
Accept limitations and understand that not everyone is a candidate for a great outcome with the BBL procedure.
Best, 
Mats Hagstrom MD