There isn't just one thing that causes a relapse.
In my case I not only became lazy with doing what I had been doing to maintain abstinence (meetings, sponsor, etc) but I was stuck on simply not drinking. I knew there was more to it than that, but I didn't know what to do to get to the next level in MY recovery. I figured that when the time was right I would know when I was ready to start doing the actual hard work, but nothing really changed.
I was 15 months clean and sober (pot and alcohol) but not "working my recovery." I was on the right path, but the way was blocked. Unfortunately I choose to remove that roadblock by sabotaging my sobriety. Fortunately I was able to get back on the right path right away without causing any more damage, and believe me, the tuition for this lesson was very costly.
I guess my recent relapse incident (one night) was something I had to go through in order for me to learn what I needed to learn to be able to move forward. Could I have moved on without the experience of this relapse incident? Possibly. I'll never know because how would I know something I never knew in the first place? I'm sure there are other ways I could have learned how to get from point A (abstinence) to point B (bridge to long term sobriety).
I kind of feel like I was in recovery kindergarten and now I've moved up a grade. Not that we ever get to graduate, because it's continuing education for life.
So a relapse isn't the end of the world. It's not an excuse to drink either. Survival is not guaranteed.