Is there a reason why people get a 2nd and 3rd BBL?

BBL revision surgery can address a variety of concerns following the initial procedure, such as improper healing, dissatisfaction with size, and asymmetrical results. Revisions may involve adding fat to correct or increase results, or removing some of the fat that was transferred during liposuction.  If the original BBL was performed with optimal results, there is no reason for a revision.
Sometimes a patient requests a second round if they gain weight or want more liposuction. Having the BBL done a second time is an option as long as the patient has enough fat to be transferred. The second round may have been part of the original plan which was to be completed in stages. The percentage of fat survival in the BBL the second time should be about the same as the first.   The volume loss occurs over the first 6 weeks or so.  After that period of time the volume remains stable assuming weight is maintained.  I always transfer a greater amount of fat to compensate.   
I perform a great number of BBL revisions from around the country and from other countries.  I would say that an important thing to determine is the revision rate of the board certified plastic surgeon you select for the BBL.  Find a board certified plastic surgeon who performs hundreds of BBLs each year and consistently produces the results you would expect. Look at reviews on this site and see who gets the surgery right every time.  Kenneth Hughes, MD, Board Certified Plastic SurgeonLos Angeles, CA

The controversy surrounding the "permanence" or "temporary" nature of the results buttock augmentation via autologous fat grafting speaks to the variable nature of fat grafting as a technique.
Technically speaking the results of fat grafting should be permanent in the sense that grafted fat which has become vascularized should not disappear several months post op.
However, there is an initial period, immediately post op where some of the grafted fat is lost. It is during this period that some may sense that their aesthetic outcome is fleeting. Every patient reaches a point after which the grafted fat which has become vascularized is permanent and the fat graft which has not will have undergone resorption. This is typically reached by 3-4 months post op. 
For some patients, the graft take vs loss ratio will be favorable and for others less so. There is no question that there is both variability in techniques as well as outcome.With regards to your specific question, the reason for repeat grafting is often times the pursuit of volume. The repeat patient may not have achieved their desired volume after their original procedure (lack of donor, lack of recipient space, resorption) or alternatively they are seeking to even out some irregularities or asymmetries. Thus whether or not you will need/want a second procedure is entirely dependent on the outcome of your initial procedure. Fortunately, your surgeon should be able to give you some idea as to whether your initial goals are in line with what you might expect to reasonably achieve.
As always, you should seek a board certified plastic surgeon (ABPS) who is experienced in both the science and art of autologous fat grafting to help you understand the procedure and provide a reasonable idea of what to expect both immediately post op and long term.

The reason why some women seek additional rounds of BBL is that, generally, they want greater volume than can be achieved with one surgery. There's a finite amount that can be achieved with one operation. Safety concerns regarding the amount of liposuction that can be performed at one procedure, as well as the pressure within the soft tissue of the buttocks, both limit the amount of fat that can be transferred. Both of these issues can be overcome by staging the fat transfer over multiple procedures.Hope this helps! Sam Jejurikar, MD