Slow Tire Leak

2000 Accord--found a near flat tire last night. Pumped it up and found that the pressure dropped about 5 psi over night. Took the tire off, first thing
I checked was the stem with a bit of water---.lucky me-- It showed a small leak. Tightened the stem and the leak stopped. Didn't want to trust it so I just replaced the stem. Just curious, what the heck would cause the stem to loosen up a bit? I've seen worn seats but never this type of problem. MLD
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