PVC Compression Coupling develops major leak after 4 weeks

I had to replace a PIA saltwater device by cutting it out between the adjoining 2" pvc pipes and replaced it with 2" pipe along with a PVC Compression Coupling. This has worked great for over 4 weeks, but suddenly it spouted a major leak at one of the screw in ends. Tightening it made no difference. I took it all apart but found nothing obviously wrong and the rubber seals looked good, so I put it all back together and it stopped leaking and has not done so for a day. I should be happy, but I would like to find out why it suddenly developed that leak. When I first checked on it I could tighten it easily by hand, but the leak still persisted and so I used a wrench to give it another quarter turn which slowed it a bit. However when I first installed it I hand hand tightened it as much as I could and that worked but after the leak I could easily turn it half turn!
What should I look for in order to prevent an other sudden major leak possibly when I cannot catch it in time?