I did not often manage much more than a week or so sober including the last time I picked up so in some respects calling it a relapse would be giving it more credit than it deserves however, to answer the question - what caused me to drink was giving in to the craving to drink. What made me not do that seven and a half years ago was avoiding the craving by doing something - walking, restoring an old car, speaking to someone on the phone, anything to distract the AV.
One other thing, the cravings only bad in the evenings so I used to get up at zero dark thirty for about 6 months in order to be tired enough to be in bed by 9.30 and dodge craving time that way.
It was an analogue method, not very clever or psychological but effective because I really didn't think I could quit.