Even if you've checked for shorts, I've found that shorts can happen with a new coil or wick as soon as you fire it. Might be due to the heat causing expansion of the coil or wick, or from the action of the vaporization (boiling of the eliquid) agitating the thin layer of oxidation. Solution I've found: oxidize the wick some more by wetting with PG, VG, or eliquid, and lighting with a match. Repeat a few times. After you've done a few of these coils, you'll have an idea of how much oxidation you need.
Popping of the coil happens when you try to get all of the wattage in that small area between the top post, and where the coil shorts to the wick (at the top). All of that wattage is supposed to be spread out over the entire coil, so that no spot would get hot enough to melt the coil. Contact with the wick will keep the coil less hot, as the heat will go into the wick as well. Most of the time, this hot spot will be at the top of the coil, since that's the spot that is most difficult to keep the coil near enough to the wick. Too tight, and you will scrape the oxidation away. Too loose, and you'll have only the coil heating up (and glowing)
When breaking in a new coil or wick, I start at the lowest voltage, and slowly work my way up to my normal vaping voltage. A drop of eliquid on it, when first firing, may delay the coil popping long enough for you to see where the hot spot is.