A large number of things could increase your chances of abnormals, age is a fairly big factor.
The thing is IMO, we end up knowing a little too much about the process when doing IVF/PGD. Falling pregnant naturally isn't all that easy either (even though many of us have been lucky to fall first try...myself included) and abnormals are very common within naturally conceived embryos...many of us would have miscarried at some point and not even been aware that we were pregnant. The difference with IVF/PGD is that we are mass producing embryos and testing them so we become aware of these chromosomal abnormalities that we would not have otherwise known about.
I would thing that there are obvious things we could do (both partners) that could potentially decrease our risks of abnormals; ie healthy lifestyle, limiting contact with chemicals, processed things etc but at the end of the day I think it is important accept that it is more than likely that you will encounter chromosomal abnormalities in your cycle. Last time out of 5 that made it to PGD we had 2 abnormals.
Some people have more abnormals than others...maybe its a luck thing and maybe its just "normal" for them.