What caused the double bubble that I've had with my breast augmentation, and what should be done to correct it? (Photo)

It appears in your case that the implants had too large of a base diameter so your surgeon released the bottom edge of the breast (inframammary fold) in order to center the implants. You may have had a slight tubular shape, with a short nipple-to-fold distance, which predisposes to this type of problem. You will likely require a correction with capsulorrhaphy to raise the fold, and switch to either smaller implants or an anatomic shape.

Looking at your photos, you do appear to have implants that are too
big for your chest. They have also been placed too low on your chest.
Thirdly, although the implants would have needed to be placed below your
orginal infra mammary crease, the constriction in the crease should
have been released internally to minimise the effect. You would be
better off with smaller implants and reconstruction of the inframammary
fold. 
For optimal outcome, please consult a fully qualified plastic surgeon, who
has specialist qualification in plastic surgery 'FRCS Plast' after having being
trained adequately in the field of plastic surgery, is registered on the
'Specialist register for Plastic surgery' with the General Medical Council and
has been considered competent enough to be a member of the 'BAPRAS' (British
Association of Plastic Reconstructive and Aesthetic Surgeons) and 'BAAPS'
(British Association of Aesthetic Plastic Surgeons) at the least.

Dear ally_davies,
though rare, some women who undergo breast augmentation will develop a minor deformity known as a “double bubble,” in which additional folds appear underneath the breasts due to the implant accidentally shifting or contracting post-surgery. In most cases, the implant simply slipped too far down the chest wall and traveled behind the breast inframammary crease (the area where the lower breast meets the chest), instead of moving forward to fill the breast cavity. If left untreated, the lopsided implant will form unnatural indentations, resulting in an odd “four-breast effect” rather than the full, rounded chest the patient initially desired.
Daniel Barrett, MD
Certified, American Board of Plastic Surgery
Member, American Society of Plastic Surgery
Member, American Society of Aesthetic Plastic Surgery