For the people in early sobriety i would say it is complacency that always gets them but for people with say 5 years plus abstinence in my experience it has been a big change in circumstances that has put them into a position they have not had to health with in their sober time. Thinking back on some of the ones i have seen: debt, new relationship, failed relationship, death, illness, change in career and stuff like that. They didn't leave a back door open and their sobriety was solid but it happened, so i guess, to coin a phrase sobriety lost its priority. I know quite a few old timers and sobriety/recovery is their life, it is above all else and they stay sober but not allowing family, friends, career, relationships etc to take over the number one spot of their priorities; not easy but achievable if you have that level of commitment and maintain it.