In addition to the obligatory "concave path of rod tip" and 'application of power" advice all instructors would (and should) give you, I would offer a third perspective gleaned from my own studies of this common problem.
First - concave path of rod tip...
If the rod starts high, dips pulling the line downward as it comes forward, then pops upward at the end, the far end of the fly line (influenced most by the earliest action, the dip) will cross and fall below the bottom leg of the loop (influenced most by the latest action, the upward pop).
In other words these two paths cross. The longer the line being aerialized, the more prone this is to occur given a rod path that is not straight or slightly open.
Application of power...
If you over-flex the rod as it starts out, the line is going to start forward at a lower level, soon after which the rod retaliates to this abuse by popping back into shape mid-stroke which puts creates essentially the same effect as the concave path - except it's more of a half moon instead of the full sickle. Same result.
Another power problem is creep, which is when the rod comes forward without any load in it before power is applied. Gravity will pull the line down, then a late hit against a slack line is going to do more than a subtle tail - most likely spaghetti will result.
Late power - usually if the line and rod are loaded correctly as it comes forward a late 'hit' or burst of power is not a problem. In fact some expert casters thrive on an explosive burst at the end of the stroke but it must be applied to a tensioned approach.
My observations on the tailing loop...
After struggling to find the common link behind these classic explanations, I came up with the simple explanation that I alluded to in the concave path paragraph - if the tension in the line is first applied in one direction (vector) than the path created by the final application of power must not pass above that path or a tailing loop will result.
Lefty Kreh says this same thing in a different way during his more recent presentations, "tuck the rod tip underneath the line".
So to have good loop form, concentrate on aligning the initial path of acceleration toward the target because that is what influences the end of the fly line and leader's turnover. Then maintain gradually increasing tension thru the stroke and make sure the final snap turns the line over so that the bottom half is beneath the initial path.
For tighter loops, make the snap and turnover as abrupt and as close to the initial path as possible.
Hope we can do some casting together soon, and not in a nor'easter!
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