A Hello, and thanks for this very good question. The most likely answer is that there was always some degree of cellulite/dimpling there to begin with, but it wasn't obvious. The increased buttock volume from your fat transfer pushing the tissue out caused these irregularities to become more obvious, and maybe even enhanced them. It is unlikely that the fat transfer actually caused cellulite. Both cellulite and dimples occur because of irregularities in the length of fibers called "septa" that run from the undersurface of the skin down to the fascia (covering of the muscle). When the fibers are all of similar lengths, the fat in between the fibers is spaced out in a smooth, uniform manner. When the fibers are of different lengths, the fat in one "compartment" bounded by fibers all around it may be lower or higher than the compartment of fat next to it. This causes the irregular, bumpy look to the skin we see in cellulite, and if the fibers are all shortened in a larger area compared to the surrounding fibers, it causes a dimple (also likely a form of cellulite). This, unfortunately, has to do with your genetics more than anything else, because even thinner people can have some cellulite.
However, exercise and hydration can improve things to some degree. There are creams and procedures involving rollers and suction that have been used for years, but results are transient, and they do not cure the condition. Various other procedures involving lasers and/or cutting of the fibers beneath the skin (to allow the fat compartments to equalize and smooth out) have also been used for years with unpredictable results.
I tell my patients (and there are many with cellulite and dimpling) that fat transfer (BBL) will make their buttock fuller and improve their figure, but it will not smooth out cellulite and dimples, and could even enhance them. I do, however, perform limited "subcision" (cutting of the shorter, tighter fibers) during fat transfer procedures where there are deeper dimples. If it is done carefully (with a small cannula with a small forked blade on the end) deeper dimples can be safely softened and partially or completely smoothed out. Too much subcision, however, could lead to irregularities that are even worse than the dimples.
There are two procedures which may have some value. One is Cellulaze, which uses a laser to cut the fibers, and another is Cellfina, which uses a wire to uniformly cut the fibers in a given area. Both are done under local anesthesia. I have no experience with these treatments, but I know they can be successful, especially in limited areas.
I hope this helps.