It can be caused by either Down's or the placenta not functioning properly, which can happen for any number of reasons and I doubt has anything to do with drinking or the odd cigarette (I've not found any data showing evidence of causation) so don't beat yourself up.
I don't think there's anything that can be done to change it unfortunately, it's a case of keeping an eye on it.
My PAPP-A was 0.4 MoM which is right on the borderline for extra monitoring at my hospital. The paper I found said that its levels of 0.15 and below that are most associated with problems. I doubt yours is that low as if it was, your Down's risk would have been higher. It's important to remember that it only means there might be a problem, not that there is one, and most people with low PAPP-A have no complications at all.
I have a scan at 23 weeks to check the flow of blood to and from the baby. Assuming that's all fine, they'll probably offer me a routine growth scan at 32-34 weeks to make sure the baby is growing ok, but I should be considered low risk. If the scan at 23 weeks indicates an issue, then I'll get extra monitoring - both for baby growth and also for problems for me associated with a poorly functioning placenta, e.g. pre-eclampsia.
I panicked too when I found out about it as the worst case scenarios are so scary, but they are worse case scenarios - I found after I did some reading I got some perspective and realised things are more than likely going to be fine, and it's actually good they picked up on it now as they can keep an eye on it. 10-15 years ago they didn't test for PAPP-A so think how many women could have had this problem and not known and not had the extra monitoring.
Long post, sorry! Hope it's helpful. Feel free to PM me with questions, although I'm no expert!